[llvm] [RISCV] Add basic scalar support for MERGE, MVM, and MVMN from P extension (PR #180677)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 60164 tests passed
* 1424 tests skipped

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

<details>
<summary>[code=4294967295] bin/llvm-split.exe</summary>

```
FAILED: [code=4294967295] bin/llvm-split.exe
cmd.exe /C "cd . && C:\BuildTools\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe -E vs_link_exe --intdir=tools\llvm-split\CMakeFiles\llvm-split.dir --rc="C:\Program Files (x86)\Windows Kits\10\bin\10.0.19041.0\x64\rc.exe" --mt="C:\Program Files (x86)\Windows Kits\10\bin\10.0.19041.0\x64\mt.exe" --manifests  -- C:\clang\clang-msvc\bin\lld-link.exe /nologo @CMakeFiles\llvm-split.rsp  /out:bin\llvm-split.exe /implib:lib\llvm-split.lib /pdb:bin\llvm-split.pdb /version:0.0 /MANIFEST:NO /STACK:10000000 /INCREMENTAL:NO /subsystem:console  && cd ."
LINK: command "C:\clang\clang-msvc\bin\lld-link.exe /nologo @CMakeFiles\llvm-split.rsp /out:bin\llvm-split.exe /implib:lib\llvm-split.lib /pdb:bin\llvm-split.pdb /version:0.0 /MANIFEST:NO /STACK:10000000 /INCREMENTAL:NO /subsystem:console" failed (exit code 1) with the following output:
lld-link: error: undefined symbol: LLVMInitializeWebAssemblyTarget
>>> referenced by tools\llvm-split\CMakeFiles\llvm-split.dir\llvm-split.cpp.obj:(void __cdecl llvm::InitializeAllTargets(void))
```
</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/180677