nvc0/ir: add fixup to deal with interpolateAtSample with non-MSAA
The spec calls to always use sample 0 in this case, whereas we can do undefined things for invalid sample id's in the MSAA case.
Fixes dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_sample.non_multisample_buffer.sample_n_*
Signed-off-by: Ilia Mirkin imirkin@alum.mit.edu