[SLP][X86] Add 32-bit vector stores to help vectorization opportunities

Building on the work on D124284, this patch tags v4i8 and v2i16 vector loads as custom, enabling SLP to try to vectorize these types ending in a partial store (using the SSE MOVD instruction) - we already do something similar for 64-bit vector types.

Differential Revision: https://reviews.llvm.org/D127604