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

[Gábor Spaits via llvm-commits]

spaits wrote:

Also related to:
https://reviews.llvm.org/D65143
And fixes the problem pointed out in:
https://github.com/llvm/llvm-project/commit/6b248fca333831baa0bc3a0de169befd2a832628

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