AMDGPU/GlobalISel: Custom legalize v2s16 G_SHUFFLE_VECTOR

Try to keep simple v2s16 cases as-is. This will more naturally map to
how the VOP3P op_sel modifiers work compared to the expansion
involving bitcasts and bitshifts.

This could maybe try harder with wider source vector types, although
that could be handled with a pre-legalize combine.