[llvm] [IR] Add llvm `clmul` intrinsic (PR #140301)

[Oscar Smith via llvm-commits]

oscardssmith wrote:

I've now added RiscV tests. I assume I need SelectionDag tests to show that the fallbacks implemented here are correct, but I don't see any guidance in https://releases.llvm.org/8.0.0/docs/ExtendingLLVM.html (or past PRs that I've looked at) on where these tests should go.

https://github.com/llvm/llvm-project/pull/140301