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>

src/compiler/nir/nir.h

@@ -148,7 +148,7 @@ typedef struct nir_constant {
     * by the type associated with the \c nir_variable.  Constants may be
     * scalars, vectors, or matrices.
     */
-   nir_const_value values[NIR_MAX_MATRIX_COLUMNS][NIR_MAX_VEC_COMPONENTS];
+   nir_const_value values[NIR_MAX_VEC_COMPONENTS];
 
    /* we could get this from the var->type but makes clone *much* easier to
     * not have to care about the type.