i965/fs: Add support for copy propagation.

i965/fs: Add support for copy propagation.

We could do more by handling abs/negate and non-GRF sources, but this is
a good start.  Improves tropics performance 0.30% +/- .17% (n=43).

shader-db results:
Total instructions: 208032 -> 207184
60/1246 programs affected (4.8%)
23286 -> 22438 instructions in affected programs (3.6% reduction)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>