vc4: Optimize out redundant SF updates.

vc4: Optimize out redundant SF updates.

Tiny change on shader-db currently, but it will be important when we start
emitting a lot of SFs from the same variable as part of control flow
support.

total instructions in shared programs: 89463 -> 89430 (-0.04%)
instructions in affected programs:     1522 -> 1489 (-2.17%)
total estimated cycles in shared programs: 250060 -> 250015 (-0.02%)
estimated cycles in affected programs:     8568 -> 8523 (-0.53%)