[llvm] [DAG] SimplifyDemandedBits - ensure we demand the high bits for shl nsw/nuw ops (PR #70041)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

We're actually very badly split in SimplifyDemandedBits - so if you have a preferred approach I'll see if I can make it more consistent:

SHL does the dropPoisonFlags approach, but only for variable shifts. I'll take your suggestion and see what happens by adding it for the constant shift as well instead of what I tried first. I'm worried that promotions and address math might be relying on NUW/NSW in some places.

SRA/SRL adds extra demanded bits for EXACT and doesn't alter flags at all.

https://github.com/llvm/llvm-project/pull/70041