[clang] [llvm] [RISCV][MC] Add experimental `Zvvmm` MC support (PR #193956)

[via cfe-commits]

imkiva wrote:

> I noticed there are so many sub-extensions in the spec but it seems we only support `Zvmm` here?

Yes, that is intentional. The IME spec contains quite a few sub-extensions, and adding all of them in a single PR would make the patch fairly large and harder to review.

I’m planning to split the support into smaller follow-up PRs, with each PR covering one sub-extension or a small group of closely related sub-extensions. This PR is meant to cover only Zvvmm, together with the common ISAInfo/vtype infra needed for it.

https://github.com/llvm/llvm-project/pull/193956