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

[Gábor Spaits via llvm-commits]

spaits wrote:

@antoniofrighetto Thank you very much for your review.
I think I have addressed all the reviews. Sorry for the amount of typos.
In the case of your last comment about ` MaxSpeculatedInstructionsToHoist` I think we should wait for other reviewers opinions too. 

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