vc4: Make SF be a flag on the QIR instructions.

vc4: Make SF be a flag on the QIR instructions.

Right now the places that used to emit a mov.sf just put the SF on the
previous instruction when it generated the source of the SF value.  Even
without optimization to push the sf up further (and kill thus potentially
kill more MOVs), this gets us:

total uniforms in shared programs: 13455 -> 13457 (0.01%)
uniforms in affected programs:     3 -> 5 (66.67%)
total instructions in shared programs: 40296 -> 40198 (-0.24%)
instructions in affected programs:     12595 -> 12497 (-0.78%)