[llvm] [InstCombine] Fold select of clamped shifts (PR #114797)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Apologies for missing some of the newer InstCombine methods - I haven't had to touch this code for a few years.

SimplifyDemandedBits would limit us to pow2 cases, but I'm not sure how important non-pow2 really is?

We could just use SimplifyMultipleUseDemandedBits - that currently works for the AND case at least.

https://github.com/llvm/llvm-project/pull/114797