vc4: Fix math with a condition flag set.

vc4: Fix math with a condition flag set.

Math results land in r4, regardless of the condition.  To implement them,
we just need to ensure that the results are moved out of r4 (as often
happens anyway, the values is live across another math instruction), so
that we can attach the condition to the MOV.

Fixes dEQP-GLES2.functional.shaders.random.all_features.fragment.93 and a
couple others, that were assertion failing that their conditions hadn't
been handled during the QIR->QPU stage.