nir/types: Add a natural size and alignment helper

nir/types: Add a natural size and alignment helper

The size and alignment are "natural" in the sense that everything is
aligned to a scalar.  This is a bit tighter than std430 where vec3s are
required to be aligned to a vec4.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>