ra: make the p, q test more efficient

ra: make the p, q test more efficient

We can store the q total that pq_test() would've calculated in the node
itself, updating it when we add a node to the stack. This way, we only
have to walk the adjacency list when we push a node on the stack (i.e.
when the p, q test succeeds) instead of every time we do the p, q test.

No difference in shader-db run times, but I'm keeping this in because
the q total that it calculates will also be used in the next few commits.

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