vc4: Introduce scheduling of QPU instructions.

This doesn't reschedule much currently, just tries to fit things into the
regfile A/B write-versus-read slots (the cause of the improvements in
shader-db), and hide texture fetch latency by scheduling setup early and
results collection late (haven't performance tested it).  This
infrastructure will be important for doing instruction pairing, though.

shader-db2 results:
total instructions in shared programs: 61874 -> 59583 (-3.70%)
instructions in affected programs:     50677 -> 48386 (-4.52%)

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