vc4: Add a constant folding pass.

vc4: Add a constant folding pass.

This cleans up some pointless operations generated by the in-driver mul24
lowering (commonly generated by making a vec4 index for a matrix in a
uniform array).

I could fill in other operations, but pretty much anything else ought to
be getting handled at the NIR level, I think.

total uniforms in shared programs: 13423 -> 13421 (-0.01%)
uniforms in affected programs:     346 -> 344 (-0.58%)