ra: don't consider nodes for spilling we don't need to

ra: don't consider nodes for spilling we don't need to

Previously, we would consider any optimistically colored nodes for
spilling. However, spilling any optimistically colored nodes below the
node that we failed to color on the stack wouldn't help us make
progress, since it wouldn't help with allowing us to find a color for
the node currently failing to get colored. Only consider nodes
which were above the failing node on the stack for spilling, which
simplifies the logic, and comment the code better so people know what's
going on here.

No shader-db changes with BRW_MAX_GRF reduced to 90 (or with the normal
number of GRF's).

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