i965: Flatten if-statements beyond depth 16 on pre-gen6.
i965: Flatten if-statements beyond depth 16 on pre-gen6.

Gen4 and Gen5 hardware can have a maximum supported nesting depth of 16.
Previously, shaders with control flow nested 17 levels deep would
cause a driver assertion or segmentation fault.

Gen6 (Sandybridge) hardware no longer has this restriction.

Fixes fd.o bug #31967.

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