i965/fs: Let sat-prop ignore live ranges if producer already has sat.

i965/fs: Let sat-prop ignore live ranges if producer already has sat.

This sequence (where both x and w are used afterwards) wasn't handled.

   mul.sat x, y, z
   ...
   mov.sat w, x

We assumed that if x was used after the mov.sat, that we couldn't
propagate the saturate modifier, but in fact x was already saturated.

So ignore the live range check if the producing instruction already
saturates its result. Cuts one instruction from hundreds of TF2 shaders.

total instructions in shared programs: 1995631 -> 1994951 (-0.03%)
instructions in affected programs:     155248 -> 154568 (-0.44%)

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