glsl: Optimize (x + y cmp 0) into (x cmp -y).
glsl: Optimize (x + y cmp 0) into (x cmp -y).

Cuts a small handful of instructions in Serious Sam 3:

instructions in affected programs:     4692 -> 4666 (-0.55%)

Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

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