i965/fs: Allow copy propagation with source modifiers.

i965/fs: Allow copy propagation with source modifiers.

This shaves a few instructions off of a ton of programs.  For 12
shaders from tropics and sanctuary, it's enough reduction in register
pressure to get 16-wide mode.  7 shaders from heroes of newerth and
savage2 are hurt by about 1.1%, where copy propagation of negates ends
up preventing coalescing, but we could regain that by doing dataflow
analysis in our copy propagation.

No significant performance difference in tropics (n=11)

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