ra: Add an adjacency list to trade space for time in ra_simplify().
ra: Add an adjacency list to trade space for time in ra_simplify().

This was recommended in the original paper, but I figued "make it run"
before "make it fast".  Now we make it fast.  Reduces the runtime of
glsl-fs-convolution-1 by 12.7% +/- 0.6% (n=5).

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