[llvm] [SimplifyCFG] Treat umul + extract pattern as one cheap single instruction (#115683) (Approach 2) (PR #128021)

[Gábor Spaits via llvm-commits]

spaits wrote:

@antoniofrighetto @dtcxzyw @goldsteinn @nikic Sorry for re-doing the PR and with that creating more review related work.
It is my bad, that I have recognized it pretty late, that this patch would be more effective in `dominatesMergePoint`.

I can promise, that the working of this PR is basically the same as the previous version. It is still what @dtcxzyw has suggested in this comment: https://github.com/llvm/llvm-project/issues/115683#issuecomment-2502401576 . But because `dominatesMergePoint` the new place of implementation only takes the cost of instruction into account and not the number of instructions it behaves better and fixes an older test.

That previously mentioned older test was created for the exact reason why the issue was reported as seen in the commits
https://github.com/llvm/llvm-project/commit/6b248fca333831baa0bc3a0de169befd2a832628 .
 
Also this version addresses @antoniofrighetto and @goldsteinn 's concerns about the ugly variables that were needed in `speculativelyExecuteBB` to still account for the hoistable instruction limit (`PartialInst` and `MaxSpeculatedInstructionsToHoist`).

@nikic now I have created better tests: there is a negative test (cost overrun) and some rather unlikely edge case (can we find and handle when the pattern is found multiple times).  

Once again, I am sorry for changing this much after the PR was published and I would be really grateful if you could re-review it.

Than you for your patience and time.

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