nir/gl_nir_lower_buffers: Set up align_mul/offset on UBOs.

nir/gl_nir_lower_buffers: Set up align_mul/offset on UBOs.

nir_lower_to_explicit_io will give us good alignments if we have the
cast's alignment information known, and it's trivial: Just the offset of
the UBO variable that is at the base of the deref.  Otherwise, explicit io
assumes the load is aligned just to the size of a scalar value in it.

The change in freedreno is in the noise.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6612>