[WebAssembly] Disable SimplifyDemandedVectorElts after legalization

This fixes a reported bug that caused an infinite loop during the
SelectionDAG optimization phase in ISel, by creating an overridable hook
in `TargetLowering` that allows us to bail out from running
`SimplifyDemandedVectorElts`.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D121869