[X86][SSE] Add general 32-bit LOAD + VZEXT_MOVL support to EltsFromConsecutiveLoads

This patch adds support for consecutive (load/undef elements) 32-bit loads, followed by trailing undef/zero elements to be combined to a single MOVD load.

Differential Revision: http://reviews.llvm.org/D16729

llvm-svn: 259796