broadcom/vc4: Use the RA callback to improve register selection's choices.

broadcom/vc4: Use the RA callback to improve register selection's choices.

We simply pick r4 if available (anything else would force a MOV), then
round-robin through accumulators (avoids physical regfile RAW delay
slots), then round-robin through the physical regfile.

The effect on instruction count is pretty impressive:

total instructions in shared programs: 76563 -> 74526 (-2.66%)
instructions in affected programs:     66463 -> 64426 (-3.06%)

and we could probably do better with a little heuristic of "if we're going
to choose a physical reg, and other operands of instructions using this as
a src have the same physical regfile, then use the other regfile".