[llvm] [RISCV][P-ext] Add codegen for packed reduction sum (PR #206004)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-mlir-rhel-clang` running on `ppc64le-mlir-rhel-test` while building `llvm` at step 5 "build-check-mlir-build-only".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/129/builds/46953

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

```
Step 5 (build-check-mlir-build-only) failure: build (failure) (timed out)
...
1152.677 [216/8/4510] Linking CXX static library lib/libLLVMipo.a
1153.202 [215/8/4511] Linking CXX static library lib/libLLVMCoroutines.a
1153.822 [214/8/4512] Linking CXX static library lib/libLLVMPasses.a
1154.458 [211/10/4513] Linking CXX static library lib/libMLIRExecutionEngineUtils.a
1154.481 [211/9/4514] Linking CXX static library lib/libLLVMOrcJIT.a
1159.136 [211/8/4515] Building CXX object tools/mlir/lib/Dialect/LLVMIR/CMakeFiles/obj.MLIRLLVMDialect.dir/IR/LLVMAttrs.cpp.o
1159.797 [211/7/4516] Linking CXX executable bin/llc
1164.730 [211/6/4517] Building CXX object tools/mlir/lib/Conversion/NVGPUToNVVM/CMakeFiles/obj.MLIRNVGPUToNVVM.dir/NVGPUToNVVM.cpp.o
1195.571 [211/5/4518] Building CXX object tools/mlir/lib/Conversion/AMDGPUToROCDL/CMakeFiles/obj.MLIRAMDGPUToROCDL.dir/AMDGPUToROCDL.cpp.o
1213.105 [211/4/4519] Building CXX object tools/mlir/lib/Conversion/MathToNVVM/CMakeFiles/obj.MLIRMathToNVVM.dir/MathToNVVM.cpp.o
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir-build-only'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2415.278026

```

</details>

https://github.com/llvm/llvm-project/pull/206004