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

[Sudharsan Veeravalli via llvm-commits]

svs-quic wrote:

Thanks Sam and Craig for your comments and approval. I think my initial patch and commit message did not explain things well. The changes in ISelLowering are only for some special cases that I have now mentioned in the commit message. 

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