mesa: Fix test for optimistic coloring being necessary.

mesa: Fix test for optimistic coloring being necessary.

i965 and radeon use ra_set_node_reg() to force payload registers to
specific registers while exposing those registers to the allocator still.
We were treating those register nodes as unsuccessfully allocated in the
ra_simplify() step, leading to walking the registers again to do
optimistic coloring even if there was nothing left ot do.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>