glsl: Ignore ir_texture in lower_const_arrays_to_uniforms.

glsl: Ignore ir_texture in lower_const_arrays_to_uniforms.

The only part of an ir_texture which can be an array is the
offsets array in textureGatherOffsets() calls.  We don't want
to lower those, because they're required to remain constants.

Fixes textureGatherOffsets with Gallium drivers such as llvmpipe,
which commit ef78df8d3b0cf540e5f08c8c2f6caa338b64a6c7 regressed.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>