[X86] Allow pre-SSE41 targets to extract multiple v16i8 elements coming from...

[X86] Allow pre-SSE41 targets to extract multiple v16i8 elements coming from the same DWORD/WORD super-element

Pre-SSE41 targets tended to have weak (serial) GPR<->VEC moves, meaning we only allowed a single v16i8 extraction before spilling the vector to stack and loading the i8 elements instead. But this didn't make use of the DWORD/WORD extraction we had to use could extract multiple i8 elements at the same time.

This patch attempts to determine if all uses of a vector are element extractions, and works out whether all the extractions share the same WORD or (lowest) DWORD, in which case we can perform a single extraction and just shift/truncate the individual elements.

Differential Revision: https://reviews.llvm.org/D156350