i965/fs: Simplify computation of key.input_slots_valid during precompile.

i965/fs: Simplify computation of key.input_slots_valid during precompile.

The for loop was rather silly.  In addition to checking brw->gen < 6
on each loop iteration, it took pains to exclude bits from
fp->Base.InputsRead that don't correspond to fragment shader inputs.
But those bits would never have been set in the first place, since the
only bits that are ever set in fp->Base.InputsRead are fragment shader
inputs.

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