nir: Make nir_constant a vector rather than a matrix

nir: Make nir_constant a vector rather than a matrix

Most places in NIR, we treat matrices like arrays.  The one annoying
exception to this has been nir_constant where a matrix is a first-class
thing.  This commit changes that so a matrix nir_constant is the same as
an array nir_constant.  This makes matrix nir_constants a tiny bit more
expensive but shrinks all others by 96B.

Reviewed-by: Karol Herbst <kherbst@redhat.com>