nv50/ir: avoid asserts when the state tracker feeds us bogus inputs

nv50/ir: avoid asserts when the state tracker feeds us bogus inputs

INTERP is defined (by me) to have to have a INPUT source. However the
state tracker does not always obey this. This happens due to varying
packing logic introducing additional mov's which can't always be undone.
Instead of just giving up, we instead try harder to find the original
input. This won't always be possible, for example with indirect
accesses. There's not much we can (easily) do about that though.

This fixes the remaining interpolateAt* failures in dEQP:

dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at*

some of which were asserting due to INTERP_* being passed a non-input.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>