glsl: teach lower_ubo_reference about samplers inside structures

glsl: teach lower_ubo_reference about samplers inside structures

In a situation like:

(tex vec4 (record_ref (var_ref f)  tex)  (constant vec2 (0.000000 0.000000))  0 1 () )

The sampler needs to be lowered, otherwise this ends up with
"ir_dereference_variable @ 0x229a100 specifies undeclared variable
`ubo_load_temp' @ 0x2290440"

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>