nir/glsl: Add an explicit_alignment field to glsl_type

nir/glsl: Add an explicit_alignment field to glsl_type

When creating explicit type, the alignment information is lost, thus
forcing explicit type users to recalculate the alignment using the same
size_align() function. Let's add a new field to cache this information.

Only structs, matrices, and vectors have and explicit alignment.  Arrays
alignment is implicitly set to its element alignment and matrices are
required to have an alignment that matches that of its vector columns.
the concept of alignment simply doesn't apply to other types.

We make the strategic choice to not allow explicit alignments on
scalars.  This is for a couple of reasons:

 1. There are no cases today where we use explicit types where we want
    any other alignment for scalars than natural alignment.

 2. Vectors don't have a component alignment that's separate from the
    explicit_alignment so it's impossible to get an explicitly aligned
    scalar type which is the component of the explicitly aligned vector
    type.

This choice may cause problems if we ever want to use explicitly laid
out types for things like varyings where we sometimes want vec4
alignment of scalars.  We can deal with that when the time comes.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6472>