[X86][AVX] Call SimplifyDemandedBits on MaskedLoadSDNode with non-boolean masks

On X86 (AVX1/AVX2), non-boolean masked loads only demand the sign bit of the mask, we already do the equivalent for masked stores.

Annoyingly I can't easily handle this inside TargetLowering::SimplifyDemandedBits as this is an x86 specific case for a generic node.

Differential Revision: https://reviews.llvm.org/D80478