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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365614 91177308-0d34-0410-b5e6-96231b3b80d8