[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 `lldb-x86_64-debian` running on `lldb-x86_64-debian` while building `llvm` at step 4 "build".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/162/builds/10436

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

```
Step 4 (build) failure: build (failure)
...
3.868 [66/37/35] Linking CXX executable bin/llvm-modextract
3.869 [66/36/36] Linking CXX executable bin/llvm-as
3.873 [66/35/37] Linking CXX executable bin/obj2yaml
3.873 [66/34/38] Linking CXX executable bin/llvm-readobj
3.885 [65/34/39] Generating ../../bin/llvm-readelf
3.887 [65/33/40] Linking CXX executable bin/llvm-link
3.893 [65/32/41] Linking CXX executable bin/llvm-pdbutil
3.895 [65/31/42] Linking CXX executable bin/verify-uselistorder
3.908 [65/30/43] Linking CXX executable bin/llvm-ifs
3.910 [65/29/44] Building CXX object lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/GlobalMergeFunctions.cpp.o
FAILED: lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/GlobalMergeFunctions.cpp.o 
/usr/bin/clang++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/worker/2.0.1/lldb-x86_64-debian/build/lib/CodeGen -I/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/lib/CodeGen -I/home/worker/2.0.1/lldb-x86_64-debian/build/include -I/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/include -fPIC -fno-semantic-interposition -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 /home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/lib/CodeGen/GlobalMergeFunctions.cpp
/home/worker/2.0.1/lldb-x86_64-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());
                                 ^
/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/lib/CodeGen/GlobalMergeFunctions.cpp:440:20: note: 'FI' declared here
    for (auto &[F, FI] : Funcs) {
                   ^
/home/worker/2.0.1/lldb-x86_64-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);
                           ^
/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/lib/CodeGen/GlobalMergeFunctions.cpp:440:20: note: 'FI' declared here
    for (auto &[F, FI] : Funcs) {
                   ^
2 errors generated.
3.924 [65/28/45] Linking CXX executable bin/llvm-profdata
3.937 [65/27/46] Linking CXX executable bin/llvm-symbolizer
3.951 [65/26/47] Linking CXX executable bin/llvm-cov
3.991 [65/25/48] Linking CXX executable bin/llvm-xray
3.998 [65/24/49] Linking CXX executable bin/clang-format
4.085 [65/23/50] Linking CXX executable bin/clang-offload-bundler
4.207 [65/22/51] Linking CXX executable bin/llvm-ml
4.220 [65/21/52] Linking CXX executable bin/llvm-dwarfdump
4.232 [65/20/53] Linking CXX executable bin/llvm-rtdyld
4.242 [65/19/54] Linking CXX executable bin/llvm-mca
4.280 [65/18/55] Linking CXX executable bin/llvm-ar
4.305 [65/17/56] Linking CXX executable bin/llvm-nm
4.305 [65/16/57] Linking CXX executable bin/llvm-debuginfo-analyzer
4.310 [65/15/58] Linking CXX executable bin/llvm-mc
4.330 [65/14/59] Linking CXX executable bin/llvm-objdump
4.334 [65/13/60] Linking CXX executable bin/sancov
4.432 [65/12/61] Linking CXX executable bin/llvm-cfi-verify
4.506 [65/11/62] Linking CXX executable bin/llvm-profgen
4.778 [65/10/63] Linking CXX shared module lib/CheckerOptionHandlingAnalyzerPlugin.so
4.798 [65/9/64] Linking CXX executable bin/diagtool
4.799 [65/8/65] Linking CXX shared module lib/CheckerDependencyHandlingAnalyzerPlugin.so
4.867 [65/7/66] Linking CXX shared module lib/SampleAnalyzerPlugin.so
5.402 [65/6/67] Linking CXX executable bin/clang-diff
5.460 [65/5/68] Linking CXX executable bin/clang-installapi

```

</details>

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