[TargetLowering] Add SimplifyDemandedBits support for ISD::INSERT_VECTOR_ELT

This helps us relax the extension of a lot of scalar elements before they are inserted into a vector.

Its exposes an issue in DAGCombiner::convertBuildVecZextToZext as some/all the zero-extensions may be relaxed to ANY_EXTEND, so we need to handle that case to avoid a couple of AVX2 VPMOVZX test regressions.

Once this is in it should be easier to fix a number of remaining failures to fold loads into VBROADCAST nodes.

Differential Revision: https://reviews.llvm.org/D59484

llvm-svn: 356989