[llvm] [SelectionDAG] Make sure demanded lanes for AND-by-zero are frozen (PR #180727)

[Björn Pettersson via llvm-commits]

bjope wrote:

I haven't tried , but maybe some of the "regressions" (if they really are regressions) here could be avoided by something like https://github.com/llvm/llvm-project/pull/145903 (i.e. preventing that recursive calls to SimplifyDemandedVectorElts from being allowed to  make certain lanes "more poisonous"). It was however a long time since I rebased those patches.
Well, that is just an idea for future improvement. I think that idea is out of scope for this fix to avoid the miscompiles reported in https://github.com/llvm/llvm-project/issues/179448 .

https://github.com/llvm/llvm-project/pull/180727