[llvm] [SelectOpt] Refactor to prepare for support more select-like operations (PR #115745)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64-aix` running on `aix-ppc64` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/64/builds/1539

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

```
Step 5 (build-unified-tree) failure: build (failure)
2.898 [134/3/1] Generating VCSRevision.h
3.065 [75/3/2] Generating VCSVersion.inc
8.713 [60/2/3] Building CXX object tools/llvm-config/CMakeFiles/llvm-config.dir/llvm-config.cpp.o
9.040 [59/2/4] Linking CXX executable bin/llvm-config
15.546 [59/1/5] Building CXX object lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/SelectOptimize.cpp.o
FAILED: lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/SelectOptimize.cpp.o 
/usr/local/clang-17.0.2/bin/clang++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_LARGE_FILE_API -D_XOPEN_SOURCE=700 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/lib/CodeGen -I/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/lib/CodeGen -I/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/include -I/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/include -mcmodel=large -fPIC -Werror -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/SelectOptimize.cpp.o -MF lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/SelectOptimize.cpp.o.d -o lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/SelectOptimize.cpp.o -c /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/lib/CodeGen/SelectOptimize.cpp
/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/lib/CodeGen/SelectOptimize.cpp:828:29: error: missing field 'Selects' initializer [-Werror,-Wmissing-field-initializers]
  828 |     SelectGroup SIGroup{Cond};
      |                             ^
1 error generated.
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/115745