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

[Gábor Spaits via llvm-commits]

spaits wrote:

@antoniofrighetto I have fixed the failing test case. It seems like we have a regression, compared to some earlier state of the PR, that has been lost to squashing. Now we get back the original state of `llvm/test/Transforms/PhaseOrdering/X86/unsigned-multiply-overflow-check.ll`. I will check why it no longer fixes that X86 test case.

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