GlobalISel: Have load lowering handle some unaligned accesses

The code for splitting an unaligned access into 2 pieces is
essentially the same as for splitting a non-power-of-2 load for
scalars. It would be better to pick an optimal memory access size and
directly use it, but splitting in half is what the DAG does.

As-is this fixes handling of some unaligned sextload/zextloads for
AMDGPU. In the future this will help drop the ugly abuse of
narrowScalar to handle splitting unaligned accesses.