vc4: Enforce one-uniform-per-instruction after optimization.

vc4: Enforce one-uniform-per-instruction after optimization.

This lets us more intelligently decide which uniform values should be put
into temporaries, by choosing the most reused values to push to temps
first.

total uniforms in shared programs: 13457 -> 13433 (-0.18%)
uniforms in affected programs:     1524 -> 1500 (-1.57%)
total instructions in shared programs: 40198 -> 40019 (-0.45%)
instructions in affected programs:     6027 -> 5848 (-2.97%)

I noticed this opportunity because with the NIR work, some programs were
happening to make different uniform copy propagation choices that
significantly increased instruction counts.