[llvm] [RISCV] Add ISel patterns for Xqciac QC.MULIADD instruction (PR #147661)

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

> I don't have a strong opinion on whether this is the right way to do this either.

Thanks for taking a look, Craig. My disposition is to merge this, given there are no objections and the codegen is working for this instruction. I realise I'm quite biased towards merging Xqci improvements.

https://github.com/llvm/llvm-project/pull/147661