[X86][SSE] EltsFromConsecutiveLoads - add basic dereferenceable support

This patch checks to see if the vector element loads are based off a dereferenceable pointer that covers the entire vector width, in which case we don't need to have element loads at both extremes of the vector width - just the start (base pointer) of it.

Another step towards partial vector loads......

Differential Revision: https://reviews.llvm.org/D64205

llvm-svn: 365614