[llvm] [AMDGPU]Refactor `lowerWaveReduce` for maintainability (PR #189223)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 3176 tests passed
* 7 tests skipped

All executed tests passed, but another part of the build **failed**. Click on a failure below to see the details.

<details>
<summary>lib/Target/AMDGPU/CMakeFiles/LLVMAMDGPUCodeGen.dir/SIISelLowering.cpp.o</summary>

```
FAILED: lib/Target/AMDGPU/CMakeFiles/LLVMAMDGPUCodeGen.dir/SIISelLowering.cpp.o
sccache /opt/llvm/bin/clang++ -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/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/Target/AMDGPU -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include -gmlt -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -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 -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17 -fvisibility=hidden -UNDEBUG -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Target/AMDGPU/CMakeFiles/LLVMAMDGPUCodeGen.dir/SIISelLowering.cpp.o -MF lib/Target/AMDGPU/CMakeFiles/LLVMAMDGPUCodeGen.dir/SIISelLowering.cpp.o.d -o lib/Target/AMDGPU/CMakeFiles/LLVMAMDGPUCodeGen.dir/SIISelLowering.cpp.o -c /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6285:42: error: captured structured bindings are a C++20 extension [-Werror,-Wc++20-extensions]
6285 |             BuildMI(BB, MI, DL, TII->get(DPPOpc), Dst).addReg(Src); // old
|                                          ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6271:13: note: 'DPPOpc' declared here
6271 |       auto [DPPOpc, ClampOpc] = getDPPOpcForWaveReduction(Opc, ST);
|             ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6286:13: error: captured structured bindings are a C++20 extension [-Werror,-Wc++20-extensions]
6286 |         if (isFPOp)
|             ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6017:23: note: 'isFPOp' declared here
6017 |     auto [is32BitOpc, isFPOp] = ClassifyWaveReductionOp(Opc);
|                       ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6299:56: error: captured structured bindings are a C++20 extension [-Werror,-Wc++20-extensions]
6299 |         auto ClampInstr = BuildMI(BB, MI, DL, TII->get(ClampOpc), Dst);
|                                                        ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6271:21: note: 'ClampOpc' declared here
6271 |       auto [DPPOpc, ClampOpc] = getDPPOpcForWaveReduction(Opc, ST);
|                     ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6300:13: error: captured structured bindings are a C++20 extension [-Werror,-Wc++20-extensions]
6300 |         if (isFPOp)
|             ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6017:23: note: 'isFPOp' declared here
6017 |     auto [is32BitOpc, isFPOp] = ClassifyWaveReductionOp(Opc);
|                       ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:6313:21: error: unused variable 'SrcWithIdentityInstr' [-Werror,-Wunused-variable]
6313 |       MachineInstr *SrcWithIdentityInstr =
|                     ^~~~~~~~~~~~~~~~~~~~
5 errors generated.
```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/189223