[llvm] [SimplifyCFG] Treat umul + extract pattern as cheap single instruction. (PR #124933)

[Gábor Spaits via llvm-commits]

spaits wrote:

I have created another solution for this problem. It is in #128021. I think that is a better one. Please check that too.

https://github.com/llvm/llvm-project/pull/124933