v3d: Add an optimization pass for redundant flags updates.

v3d: Add an optimization pass for redundant flags updates.

Our exec masking introduces lots of redundant flags updates, and even
without that there will be cases where NIR comparisons on the same sources
for different reasons may generate the same comparison instruction before
the selection.

total instructions in shared programs: 6492930 -> 6460934 (-0.49%)
total uniforms in shared programs: 2117460 -> 2115106 (-0.11%)
total spills in shared programs: 4983 -> 4987 (0.08%)
total fills in shared programs: 6408 -> 6416 (0.12%)