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

[Gábor Spaits via llvm-commits]

spaits wrote:

I think I will try to move this optimization to to `foldTwoEntryPHINode`. That function is calling the `dominatesMergePoint` function. In that I will match to the pattern and set a low cost value for this pattern.

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