[llvm] [InstCombineCompare] Use known bits to insert assume intrinsics. (PR #96017)

[via llvm-commits]

goldsteinn wrote:

 
> Yes, I've read this in IR reference too. But in theory, do you think this is a right thing to do? Or is there some principle problem with using assume intrinsics that can not be fixed? If you can point out some issue with assume intrinsics I can work on it in another MR.
> 

It creates extra uses: https://godbolt.org/z/xecq1YEda

which inturn block a fair number of optimizations.

We could theoretically add some infrastructure for discounting `assume` uses (or maybe simpler in this case, to only do this if `%x` is not an instruction or has more than some threshold of uses where we don't expect it to ever become single-use).

https://github.com/llvm/llvm-project/pull/96017