[llvm] [SimplifyCFG] Treat umul + extract pattern as one cheap single instruction (PR #128021)

[Gábor Spaits via llvm-commits]

spaits wrote:

@dtcxzyw Thank you very much for reviewing my PR. I have addressed your comments:
- Moving the new test file
- Naming values
Ready for re-review.

https://github.com/llvm/llvm-project/pull/128021