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

[Jay Foad via llvm-commits]

jayfoad wrote:

> This is my first LLVM PR, so please tell me everything that I've messed up.

The title :) Should be "clmul" not "cmul".

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