glsl: Make lower_const_arrays_to_uniforms work directly on constants.

glsl: Make lower_const_arrays_to_uniforms work directly on constants.

There's really no point in looking at ir_dereference_array of a
constant.  It also misses cases like:

  (assign () (var_ref tmp) (constant (array ...) ...))

No changes in shader-db, but keeps it working after the next commit.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>