[LegalizeVectorTypes] Create correct memoperands in SplitVecRes_INSERT_SUBVECTOR.

Previously this code just used a default constructed
MachinePointerInfo. But we know the accesses are to a fixed stack
object or at least somewhere on the stack.

While there fix the alignment passed to the full vector load/stores.

I don't think this function is currently exercised in tree so I
don't know how to test it. I just noticed it when I removed
non-constant index support in this function.

Differential Revision: https://reviews.llvm.org/D80058