vc4: Replace the SSA-style SEL operators with conditional MOVs.

vc4: Replace the SSA-style SEL operators with conditional MOVs.

I'm moving away from QIR being SSA (since NIR is doing lots of SSA
optimization for us now) and instead having QIR just be QPU operations
with virtual registers.  By making our SELs be composed of two MOVs, we
could potentially coalesce the registers for the MOV's src and dst and
eliminate the MOV.

total instructions in shared programs: 88448 -> 88028 (-0.47%)
instructions in affected programs:     39845 -> 39425 (-1.05%)
total estimated cycles in shared programs: 246306 -> 245762 (-0.22%)
estimated cycles in affected programs:     162887 -> 162343 (-0.33%)