nir: Add an alignment to nir_intrinsic_load_constant

nir: Add an alignment to nir_intrinsic_load_constant

In f1883cc73d4 we tried to pass through alignments from load_constant
intrinsics when rewriting them to load_ubo in iris.  However, those
intrinsics don't have ALIGN_MUL or ALIGN_OFFSET indices.  It's easy
enough to add them.  We just call the size/align function on the vector
type at the end of our deref chain and use the alignment returned from
there.  It's possible we could do better by walking the whole deref
chain but this should be good enough.

Fixes: f1883cc73d4 "iris: Set alignments on cbuf0 and constant reads"
Closes: #2739
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4468>