Fix PR17546

- Type of index used in extract_vector_elt or insert_vector_elt supposes
  to be TLI.getVectorIdxTy() which is pointer type on most targets. It'd
  better to truncate (or zero-extend in case it's changed later) it to
  mask element type to guarantee they are matching instead of asserting
  that.

llvm-svn: 192722