GlobalISel: Use extension instead of merge with undef in common case

This fixes not respecting signext/zeroext in these cases. In the
anyext case, this avoids a larger merge with undef and should be a
better canonical form.

This should also handle this if a merge is needed, but I'm not aware
of a case where that can happen. In a future change this will also
allow AMDGPU to drop some custom code without introducing regressions.