[llvm] [CodeGen] Refactor rematerializer unit tests to reduce boilerplate (NFC) (PR #197575)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-m68k-linux-cross` running on `suse-gary-m68k-cross` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/27/builds/11

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

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
...
[68/83] Linking CXX executable bin/Kaleidoscope-Ch5
[69/83] Linking CXX executable tools/clang/tools/extra/unittests/clang-tidy/ClangTidyTests
[70/83] Linking CXX executable bin/Kaleidoscope-Ch4
[71/83] Linking CXX executable bin/Kaleidoscope-Ch6
[72/83] Linking CXX executable unittests/tools/llvm-mca/LLVMMCATests
[73/83] Linking CXX executable unittests/tools/llvm-exegesis/LLVMExegesisTests
[74/83] Linking CXX executable bin/Kaleidoscope-Ch7
[75/83] Building CXX object unittests/ObjectYAML/CMakeFiles/ObjectYAMLTests.dir/DXContainerYAMLTest.cpp.o
[76/83] Linking CXX executable unittests/ObjectYAML/ObjectYAMLTests
[77/83] Building CXX object unittests/CodeGen/CMakeFiles/CodeGenTests.dir/RematerializerTest.cpp.o
FAILED: unittests/CodeGen/CMakeFiles/CodeGenTests.dir/RematerializerTest.cpp.o 
/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/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/unittests/CodeGen -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/unittests/CodeGen -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/include -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/include -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/third-party/unittest/googletest/include -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/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-maybe-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/CodeGen/CMakeFiles/CodeGenTests.dir/RematerializerTest.cpp.o -MF unittests/CodeGen/CMakeFiles/CodeGenTests.dir/RematerializerTest.cpp.o.d -o unittests/CodeGen/CMakeFiles/CodeGenTests.dir/RematerializerTest.cpp.o -c /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/unittests/CodeGen/RematerializerTest.cpp
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/unittests/CodeGen/RematerializerTest.cpp: In static member function ‘static void {anonymous}::RematerializerTest::SetUpTestCase()’:
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/unittests/CodeGen/RematerializerTest.cpp:96:5: error: ‘LLVMInitializeAMDGPUTargetInfo’ was not declared in this scope; did you mean ‘LLVMInitializeM68kTargetInfo’?
   96 |     LLVMInitializeAMDGPUTargetInfo();
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |     LLVMInitializeM68kTargetInfo
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/unittests/CodeGen/RematerializerTest.cpp:97:5: error: ‘LLVMInitializeAMDGPUTarget’ was not declared in this scope; did you mean ‘LLVMInitializeM68kTarget’?
   97 |     LLVMInitializeAMDGPUTarget();
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~
      |     LLVMInitializeM68kTarget
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/unittests/CodeGen/RematerializerTest.cpp:98:5: error: ‘LLVMInitializeAMDGPUTargetMC’ was not declared in this scope; did you mean ‘LLVMInitializeM68kTargetMC’?
   98 |     LLVMInitializeAMDGPUTargetMC();
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |     LLVMInitializeM68kTargetMC
[78/83] Linking CXX executable tools/clang/unittests/Interpreter/ExceptionTests/ClangReplInterpreterExceptionTests
[79/83] Linking CXX executable tools/clang/unittests/Interpreter/ClangReplInterpreterTests
[80/83] Linking CXX executable tools/clang/tools/extra/clangd/unittests/ClangdTests
[81/83] Linking CXX executable tools/clang/unittests/AllClangUnitTests
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/197575