[SelectionDAG][x86] limit post-legalization store merging by type

The proposal in D62498 showed that x86 would benefit from vector
store splitting, but that may conflict with the generic DAG
combiner's store merging transforms.

Add memory type to the existing TLI hook that enables the merging
transforms, so we can limit those changes to scalars only for x86.

llvm-svn: 362507