ra: optimistically color only one node at a time

ra: optimistically color only one node at a time

Before, when we encountered a situation where we had to optimistically
color a node, we would immediately give up and push all the remaining
nodes on the stack in the order of their index - which is a random, and
potentially not optimal, order. Instead, choose one node to
optimistically color in ra_select(), and then once we've optimistically
colored it, keep on going as normal in the hopes that we've opened up
more avenues for the normal select phase to make progress. In cases with
high register pressure, this helps make the order we push things on the
stack much better, and therefore increase the chance that we can allocate
successfully.

total instructions in shared programs: 4545447 -> 4545401 (-0.00%)
instructions in affected programs:     1353 -> 1307 (-3.40%)
GAINED:                                124
LOST:                                  6

Signed-off-by: Connor Abbott <connor.abbott@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>