[X86][SSE] Add zero element and general 64-bit VZEXT_LOAD support to EltsFromConsecutiveLoads

This patch adds support for trailing zero elements to VZEXT_LOAD loads (and checks that no zero elts occur within the consecutive load).

It also generalizes the 64-bit VZEXT_LOAD load matching to work for loads other than 2x32-bit loads.

After this patch it will also be easier to add support for other basic load patterns like 32-bit VZEXT_LOAD loads, PMOVZX and subvector load insertion.

Differential Revision: http://reviews.llvm.org/D16217

llvm-svn: 258798