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 <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

mapbox/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,dellis1972/glsl-optimizer,metora/MesaGLSLCompiler,djreep81/glsl-optimizer,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,mcanthony/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,metora/MesaGLSLCompiler,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,metora/MesaGLSLCompiler,zeux/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,mcanthony/glsl-optimizer,wolf96/glsl-optimizer,mcanthony/glsl-optimizer