AMDGPU: Don't assert on misaligned DS read2/write2 offsets

This would assert with unaligned DS access enabled. The offset may not
be aligned. Theoretically the pattern predicate should check the
memory alignment, although it is possible to have the memory be
aligned but not the immediate offset.

In this case I would expect it to use ds_{read|write}_b64 with
unaligned access, but am not clear if there's a reason it doesn't.