[llvm] [InstSimplify] Fold `select(Y >=s C, sadd.sat(X, Y) >=s X, true)` to `true` when C >… (PR #203639)

[Le Gia Bao via llvm-commits]

L3GiaBao wrote:

> This either needs proof of [real-world usefulness](https://llvm.org/docs/InstCombineContributorGuide.html#real-world-usefulness), or a more principled fold.
> 
> If we want to handle this, we should probably start by handling simpler cases first. We're currently missing this fold: https://alive2.llvm.org/ce/z/njE8Hu Specifically, this is not folded even if %y is a constant: https://llvm.godbolt.org/z/3W6eadK7M

Thanks for working on a Sunday :) I'll pick up on Tue.

https://github.com/llvm/llvm-project/pull/203639