nv50/ir: fix quadop emission in the presence of predication

nv50/ir: fix quadop emission in the presence of predication

When there's a predicate, it just goes onto the sources list. If the
quadop only has a single regular source, we will end up thinking that
the predicate is the second source. Check explicitly for the predSrc so
that we don't accidentally emit the wrong thing.

This fixes a bunch of dEQP-GLES3.functional.shaders.derivate.* tests.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org