[SelectionDAG] Improve SimplifyDemandedBits to SimplifyDemandedVectorElts simplification

D52935 introduced the ability for SimplifyDemandedBits to call SimplifyDemandedVectorElts through BITCASTs if the demanded bit mask entirely covered the sub element.

This patch relaxes this to demanding an element if we need any bit from it.

Differential Revision: https://reviews.llvm.org/D54761

llvm-svn: 348073