[clang] [llvm] [RISCV] Add MC layer support for XSfmm*. (PR #133031)

[Jiongjia Lu via cfe-commits]

Avimitin wrote:

I still get compiler error for the above case, but with additional error message:

```text
fatal error: error in backend: Invalid size request on a scalable vector.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: /nix/store/n44qhrqxb3qnbylypsfq75jl2lfsvbp6-clang-21.0.0-unstable-2025-03-23/bin/clang @/tmp/nix-shell.9YUR38/cc-params.lXn6o1
1.      <eof> parser at end of file
2.      Code generation
3.      Running pass 'Function Pass Manager' on module 'test.c'.
4.      Running pass 'RISC-V DAG->DAG Pattern Instruction Selection' on function '@utf8_count_SWAR_popc_bithack_autovec'
```

And here is the debug files:

[test.c.txt](https://github.com/user-attachments/files/20348260/test.c.txt)
[test.sh.txt](https://github.com/user-attachments/files/20348266/test.sh.txt)


https://github.com/llvm/llvm-project/pull/133031