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

[Craig Topper via cfe-commits]

topperc wrote:

> > Is the issue really related to this PR? Why would a MC change cause CodeGen errors? Is it because uncommon extension combinations?
> 
> I've played around with the -march flag a bit more, and it doesn't seem to be the main culprit here. For example, even using `-march=rv32gc_zve32f_xsfmm128t` still leads to the same crash.
> 
> However, I've found that the optimization level seems to be the key factor. The crash happens when I use -O3 or -O2. But if I switch to -O1, -Og, or just don't specify an optimization level (which defaults to -O0), the code compiles successfully without any errors.

I was able to reproduce the failure using the commit from when this PR was created. The later merge seems to have hid it. The failure still reproduces on trunk with the same IR file though.

The problem is in `combineToVCPOP` when Zve32* is enabled but not Zve64* or V*. I have a patch I'll post in a few minutes.

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