[SelectionDAG][X86] Enable iX SimplifyDemandedBits to vXi1...

[SelectionDAG][X86] Enable iX SimplifyDemandedBits to vXi1 SimplifyDemandedVectorElts simplification. Add a hack to X86 to avoid a regression

Patch showing the effect of enabling bool vector oversimplification.

Non-VLX builds can simplify a kshift shuffle, but VLX builds simplify:

insert_subvector v8i zeroinitializer, v2i --> insert_subvector v8i undef, v2i

Preventing the removal of the AND to clear the upper bits of result

Differential Revision: https://reviews.llvm.org/D53022

llvm-svn: 369780