glsl: Avoid division-by-zero during constant-folding
glsl: Avoid division-by-zero during constant-folding

Avoid division-by-zero when constant-folding the following expression
types:
    ir_unop_rsq
    ir_binop_div
    ir_binop_mod

Fixes bugs:
https://bugs.freedesktop.org//show_bug.cgi?id=33306
https://bugs.freedesktop.org//show_bug.cgi?id=33508

Fixes Piglit tests:
glslparsertest/glsl2/div-by-zero-01.frag
glslparsertest/glsl2/div-by-zero-02.frag
glslparsertest/glsl2/div-by-zero-03.frag
glslparsertest/glsl2/modulus-zero-01.frag
glslparsertest/glsl2/modulus-zero-02.frag

NOTE: This is a candidate for the 7.9 and 7.10 branches.

dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,zeux/glsl-optimizer,metora/MesaGLSLCompiler,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,KTXSoftware/glsl2agal,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,KTXSoftware/glsl2agal,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,metora/MesaGLSLCompiler,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,adobe/glsl2agal,tokyovigilante/glsl-optimizer,zz85/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,KTXSoftware/glsl2agal,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,mapbox/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,KTXSoftware/glsl2agal,zeux/glsl-optimizer,djreep81/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,adobe/glsl2agal,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,adobe/glsl2agal,djreep81/glsl-optimizer,adobe/glsl2agal,adobe/glsl2agal,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,zz85/glsl-optimizer