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

[Gábor Spaits via llvm-commits]

spaits wrote:

@goldsteinn Thank you very much for your review.
I have added the `MaxSpeculatedInstructionsToHoist` variable as you have suggested.
I kept the `PatternFound`. It is used to decide whether we should take in account the calculated cost of the basic block. I could use the `MaxSpeculatedInstructionsToHoist` variable to do this now: if the value of this is one, then there is no cheap patter, else there is. But, what if we would want to hoist more instructions than one, even if there is no patter? For that reason I keep an own variable for patterns.

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