[llvm] [CGData] Refactor Global Merge Functions (PR #115750)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-expensive-checks-debian` running on `gribozavr4` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/16/builds/8834

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
9.259 [992/33/2976] Building AArch64GenSubtargetInfo.inc...
9.282 [992/32/2977] Building OCaml library llvm_irreader
9.287 [991/32/2978] Building OCaml library llvm_bitreader
9.330 [990/32/2979] Building OCaml documentation for llvm_irreader
9.338 [989/32/2980] Building OCaml documentation for llvm_bitreader
9.339 [988/32/2981] Running utility command for ocaml_llvm_irreader
9.347 [988/31/2982] Running utility command for ocaml_llvm_bitreader
9.390 [988/30/2983] Building OCaml stub object file debuginfo_ocaml.o
9.462 [987/30/2984] Building OCaml library llvm_debuginfo
9.489 [986/30/2985] Building CXX object lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/GlobalMergeFunctions.cpp.o
FAILED: lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/GlobalMergeFunctions.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes /usr/bin/ccache /usr/bin/clang++ -DEXPENSIVE_CHECKS -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_DEBUG -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/b/1/llvm-clang-x86_64-expensive-checks-debian/build/lib/CodeGen -I/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/lib/CodeGen -I/b/1/llvm-clang-x86_64-expensive-checks-debian/build/include -I/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/include -U_GLIBCXX_DEBUG -fPIC -fvisibility-inlines-hidden -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  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -std=c++17 -MD -MT lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/GlobalMergeFunctions.cpp.o -MF lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/GlobalMergeFunctions.cpp.o.d -o lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/GlobalMergeFunctions.cpp.o -c /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/lib/CodeGen/GlobalMergeFunctions.cpp
/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/lib/CodeGen/GlobalMergeFunctions.cpp:450:34: error: reference to local binding 'FI' declared in enclosing function 'llvm::GlobalMergeFunc::merge'
              assert(InstIndex < FI.IndexInstruction->size());
                                 ^
/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/lib/CodeGen/GlobalMergeFunctions.cpp:440:20: note: 'FI' declared here
    for (auto &[F, FI] : Funcs) {
                   ^
/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/lib/CodeGen/GlobalMergeFunctions.cpp:451:28: error: reference to local binding 'FI' declared in enclosing function 'llvm::GlobalMergeFunc::merge'
              auto *Inst = FI.IndexInstruction->lookup(InstIndex);
                           ^
/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/lib/CodeGen/GlobalMergeFunctions.cpp:440:20: note: 'FI' declared here
    for (auto &[F, FI] : Funcs) {
                   ^
2 errors generated.
9.563 [986/29/2986] Building X86GenDAGISel.inc...
9.617 [986/28/2987] Building OCaml documentation for llvm_debuginfo
10.115 [986/27/2988] Building X86GenSubtargetInfo.inc...
10.337 [986/26/2989] Building AArch64GenInstrInfo.inc...
11.970 [986/25/2990] Building X86GenInstrInfo.inc...
12.345 [986/24/2991] Building RISCVGenInstrInfo.inc...
12.724 [986/23/2992] Building AMDGPUGenCallingConv.inc...
13.367 [986/22/2993] Building CXX object lib/MC/MCParser/CMakeFiles/LLVMMCParser.dir/AsmParser.cpp.o
13.609 [986/21/2994] Building AMDGPUGenPreLegalizeGICombiner.inc...
13.803 [986/20/2995] Building AMDGPUGenMCPseudoLowering.inc...
13.824 [986/19/2996] Building AMDGPUGenPostLegalizeGICombiner.inc...
14.195 [986/18/2997] Building AMDGPUGenSubtargetInfo.inc...
14.298 [986/17/2998] Building AMDGPUGenMCCodeEmitter.inc...
14.338 [986/16/2999] Building RISCVGenGlobalISel.inc...
14.687 [986/15/3000] Building AMDGPUGenRegBankGICombiner.inc...
14.701 [986/14/3001] Building AMDGPUGenDisassemblerTables.inc...
14.912 [986/13/3002] Building AMDGPUGenSearchableTables.inc...
16.287 [986/12/3003] Building RISCVGenDAGISel.inc...
19.811 [986/11/3004] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
20.663 [986/10/3005] Building RISCVGenSubtargetInfo.inc...
22.135 [986/9/3006] Building X86GenAsmMatcher.inc...
22.183 [986/8/3007] Building CXX object lib/LTO/CMakeFiles/LLVMLTO.dir/LTO.cpp.o
24.682 [986/7/3008] Building AMDGPUGenDAGISel.inc...
26.303 [986/6/3009] Building AMDGPUGenAsmWriter.inc...

```

</details>

https://github.com/llvm/llvm-project/pull/115750