vc4: Reserve rb31 instead of r3 for raddr conflict spills.

vc4: Reserve rb31 instead of r3 for raddr conflict spills.

This increases the cost of a raddr b conflict spill (save r3 to rb31, move
src1 to r3, move rb31 back to r3 when done, instead of just move src1 to
r3), but on average thanks to instruction pairing it's more worthwhile to
have another accumulator.

total instructions in shared programs: 46428 -> 46171 (-0.55%)
instructions in affected programs:     38030 -> 37773 (-0.68%)