glsl: make sure that interpolateAt arguments are variables

glsl: make sure that interpolateAt arguments are variables

In the case of a constant, it might have been propagated through and
variable_referenced() returns NULL. Error out in that case.

Fixes 3 dEQP tests:

dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_sample.negative.interpolate_constant
dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_centroid.negative.interpolate_constant
dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_offset.negative.interpolate_constant

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>