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

[Gábor Spaits via llvm-commits]

spaits wrote:

@dtcxzyw @nikic Thank you very much for your review.
I have made some changes on the PR description. Is it proper for a commit message?
I have also fixes the pattern length and the instruction counting issue.
Also added a test with some more `add` instructions in the basic block, to see that we are not hoisting, if there are more instructions.
The tests still need a deeper look. I will do more work on them soon. When is use `%add2` in the `%99 = phi i32 ...` the hoisting will happen. This is interesting, because when I do debugging I see the `return false`, when the instruction count hits 3. Maybe these instructions are being hoisted by another part of the SimplifyCFG pass?

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