[X86] Make a code path in EltsFromConsecutiveLoads work only on vectors it expects

EltsFromConsecutiveLoads was apparently only ever called for 128-bit vectors, and assumed this implicitly. r223518 started calling it for AVX-sized vectors, causing the code path that had this assumption to crash.
This adds a check to make this path fire only for 128-bit vectors.

Differential Revision: http://reviews.llvm.org/D6579