SimplifyDemandedVectorElts for all intrinsics

The point is that this simplifies integration of new intrinsics into SimplifiedDemandedVectorElts, and ensures we don't miss any existing ones.

This is intended to be NFC-ish, but as seen from the diffs, can produce slightly different output.  This is due to order of transforms w/in instcombine resulting in two slightly different fixed points.  That's something we should fix, but isn't a problem w/this patch per se.

Differential Revision: https://reviews.llvm.org/D57398

llvm-svn: 352653