ra: Take advantage of the adjacency list in finding a node to spill.

ra: Take advantage of the adjacency list in finding a node to spill.

This revealed a bug in ra_get_spill_benefit where we only considered
the benefit of the first adjacency we were to remove, explaining some
of the ugly spilling I've seen in shaders.  Because of the reduced
spilling, it reduces the runtime of glsl-fs-convolution-1 36.9% +/-
0.9% (n=5).