vc4: Allow dead code elimination of unused varyings.

vc4: Allow dead code elimination of unused varyings.

total instructions in shared programs: 39022 -> 37341 (-4.31%)
instructions in affected programs:     26979 -> 25298 (-6.23%)
total uniforms in shared programs: 11242 -> 10523 (-6.40%)
uniforms in affected programs:     5836 -> 5117 (-12.32%)