i965/fs: Do more register coalescing by using the interference graph.

i965/fs: Do more register coalescing by using the interference graph.

By using the live variables code for determining interference, we can
handle coalescing in the presence of control flow, which the other
register coalescing path couldn't.

Total instructions: 207184 -> 206990
74/1246 programs affected (5.9%)
33993 -> 33799 instructions in affected programs (0.6% reduction)

There is a newerth shader that loses out, because of some extra MOVs
that now get their dead-code nature obscured by coalescing.  This
should be fixed by doing better at dead code elimination.