glsl: Treat ir_dereference_array of non-var as a constant for lowering

glsl: Treat ir_dereference_array of non-var as a constant for lowering

Previously the code would just look at deref->array->type to see if it
was a constant.  This isn't good enough because deref->array might be
another ir_dereference_array... of a constant.  As a result,
deref->array->type wouldn't be a constant, but
deref->variable_referenced() would return NULL.  The unchecked NULL
pointer would shortly lead to a segfault.

Instead just look at the return of deref->variable_referenced().  If
it's NULL, assume that either a constant or some other form of
anonymous temporary storage is being dereferenced.

This is a bit hinkey because most drivers treat constant arrays as
uniforms, but the lowering pass treats them as temporaries.  This
keeps the behavior of the old code, so this change isn't making things
worse.

Fixes i965 piglit:

    vs-temp-array-mat[234]-index-col-rd
    vs-temp-array-mat[234]-index-col-row-rd
    vs-uniform-array-mat[234]-index-col-rd
    vs-uniform-array-mat[234]-index-col-row-rd

Reviewed-by: Eric Anholt <eric@anholt.net>