[llvm] [SimplifyCFG] Treat umul + extract pattern as one cheap single instruction (PR #128021)

[Gábor Spaits via llvm-commits]

spaits wrote:

Thank you very much @dtcxzyw for reviewing my PR. I will wait a few day before merging, so in case the other reviewers have any observations they can mention that.

https://github.com/llvm/llvm-project/pull/128021