nir: Add some sanity assertions in opt_large_constants

nir: Add some sanity assertions in opt_large_constants

We make some assumptions in opt_large_constants such as the size_align
function returning the obvious sizes for vectors.  Now that we've got
the deref_size lying around, we may as well assert it's consistent with
our assumptions.  In particular, we now assert that it really claims
booleans are 32-bit.  If anyone's driver ever decides to be clever and
change this, we'll now catch the breakage earlier.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4468>