vc4: Pair up QPU instructions when scheduling.

vc4: Pair up QPU instructions when scheduling.

We've got two mostly-independent operations in each QPU instruction, so
try to pack two operations together.  This is fairly naive (doesn't track
read and write separately in instructions, doesn't convert ADD-based MOVs
into MUL-based movs, doesn't reorder across uniform loads), but does show
a decent improvement on shader-db-2.

total instructions in shared programs: 59583 -> 57651 (-3.24%)
instructions in affected programs:     47361 -> 45429 (-4.08%)