[llvm] [KnownFPClass] Refactor direct access to `KnownFPClass::SignBit` [NFC] (PR #218514)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `intel-sycl-gpu` running on `intel-sycl-gpu-01` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/225/builds/17213

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
...
  of CMake required to build LLVM will become 3.31.0, and using an older
  CMake will become an error.  Please upgrade your CMake to at least 3.31.0
  now to avoid issues in the future.


Configuring libsycl tests
-- Configuring done (0.6s)
-- Generating done (0.2s)
-- Build files have been written to: /home/test-user/llvm-buildbot-worker/intel-sycl-gpu/build/runtimes/runtimes-bins
2.122 [20/71/1220] Building CXX object unittests/Support/CMakeFiles/SupportTests.dir/KnownFPClassTest.cpp.o
FAILED: unittests/Support/CMakeFiles/SupportTests.dir/KnownFPClassTest.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros /usr/bin/ccache /usr/bin/c++ -DLLVM_BUILD_STATIC -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/build/unittests/Support -I/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support -I/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/build/include -I/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/include -I/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include -I/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googlemock/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-dangling-reference -Wno-redundant-move -Wno-pessimizing-move -Wno-array-bounds -Wno-stringop-overread -Wno-dangling-pointer -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17 -UNDEBUG -Wno-dangling-else -Wno-variadic-macros -fno-exceptions -funwind-tables -fno-rtti -Wno-suggest-override -MD -MT unittests/Support/CMakeFiles/SupportTests.dir/KnownFPClassTest.cpp.o -MF unittests/Support/CMakeFiles/SupportTests.dir/KnownFPClassTest.cpp.o.d -o unittests/Support/CMakeFiles/SupportTests.dir/KnownFPClassTest.cpp.o -c /home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support/KnownFPClassTest.cpp
In file included from /home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:72,
                 from /home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support/KnownFPClassTest.cpp:14:
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support/KnownFPClassTest.cpp: In function ‘void {anonymous}::expectConstant(const char*, const char*, const llvm::fltSemantics&, llvm::APInt, llvm::FPClassTest, bool)’:
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support/KnownFPClassTest.cpp:34:29: error: ‘struct llvm::KnownFPClass’ has no member named ‘SignBit’; did you mean ‘getSignBit’?
   34 |   EXPECT_EQ(Negative, Known.SignBit);
      |                             ^~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include/gtest/gtest_pred_impl.h:79:52: note: in definition of macro ‘GTEST_ASSERT_’
   79 |   if (const ::testing::AssertionResult gtest_ar = (expression)) \
      |                                                    ^~~~~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include/gtest/gtest_pred_impl.h:144:3: note: in expansion of macro ‘GTEST_PRED_FORMAT2_’
  144 |   GTEST_PRED_FORMAT2_(pred_format, v1, v2, GTEST_NONFATAL_FAILURE_)
      |   ^~~~~~~~~~~~~~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:1869:3: note: in expansion of macro ‘EXPECT_PRED_FORMAT2’
 1869 |   EXPECT_PRED_FORMAT2(::testing::internal::EqHelper::Compare, val1, val2)
      |   ^~~~~~~~~~~~~~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support/KnownFPClassTest.cpp:34:3: note: in expansion of macro ‘EXPECT_EQ’
   34 |   EXPECT_EQ(Negative, Known.SignBit);
      |   ^~~~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support/KnownFPClassTest.cpp: In member function ‘virtual void {anonymous}::KnownFPClassTest_BitcastExhaustiveIEEEHalf_Test::TestBody()’:
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support/KnownFPClassTest.cpp:47:24: error: ‘struct llvm::KnownFPClass’ has no member named ‘SignBit’; did you mean ‘getSignBit’?
   47 |     ASSERT_EQ(Expected.SignBit, Known.SignBit) << RawBits;
      |                        ^~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include/gtest/gtest_pred_impl.h:79:52: note: in definition of macro ‘GTEST_ASSERT_’
   79 |   if (const ::testing::AssertionResult gtest_ar = (expression)) \
      |                                                    ^~~~~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include/gtest/gtest_pred_impl.h:148:3: note: in expansion of macro ‘GTEST_PRED_FORMAT2_’
  148 |   GTEST_PRED_FORMAT2_(pred_format, v1, v2, GTEST_FATAL_FAILURE_)
      |   ^~~~~~~~~~~~~~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:1882:3: note: in expansion of macro ‘ASSERT_PRED_FORMAT2’
 1882 |   ASSERT_PRED_FORMAT2(::testing::internal::EqHelper::Compare, val1, val2)
      |   ^~~~~~~~~~~~~~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:1898:31: note: in expansion of macro ‘GTEST_ASSERT_EQ’
 1898 | #define ASSERT_EQ(val1, val2) GTEST_ASSERT_EQ(val1, val2)
      |                               ^~~~~~~~~~~~~~~
/home/test-user/llvm-buildbot-worker/intel-sycl-gpu/llvm-project/llvm/unittests/Support/KnownFPClassTest.cpp:47:5: note: in expansion of macro ‘ASSERT_EQ’
   47 |     ASSERT_EQ(Expected.SignBit, Known.SignBit) << RawBits;
      |     ^~~~~~~~~

```

</details>

https://github.com/llvm/llvm-project/pull/218514