glsl: be more strict when validating shader inputs

glsl: be more strict when validating shader inputs

interpolateAt* can only take input variables or an element of an input
variable array. No structs.

Further, GLSL 4.40 relaxes the requirement to allow swizzles, so enable
that as well.

This fixes the following dEQP tests:

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

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>