[llvm] [CVP] Implement type narrowing for LShr (PR #119577)

[via llvm-commits]

adam-bzowski wrote:

> We already have various optimizations that do this, e.g. see canEvaluateTruncated in InstCombine and TruncInstCombine in AggressiveInstCombine. I think the reason they don't work for your sample is that computeKnownBits probably fails to determine that the top bits are zero in your sample. Probably it's just a matter of adding support for isSignedMinMaxIntrinsicClamp in computeKnownBits. (If we didn't fold the ashr to lshr it would probably also work via ComputeNumSignBits, which does handle the clamp pattern.)

I'll have a look there tomorrow and see if I can implement the narrowing in TruncInstCombine. I agree that it's not specific to lshr. Thanks!

https://github.com/llvm/llvm-project/pull/119577