linker: Improve handling of unread/unwritten shader inputs/outputs
linker: Improve handling of unread/unwritten shader inputs/outputs

Previously some shader input or outputs that hadn't received location
assignments could slip through.  This could happen when a shader
contained user-defined varyings and was used with either
fixed-function or assembly shaders.

See the piglit tests glsl-[fv]s-user-varying-ff and
sso-user-varying-0[12].

NOTE: this is a candidate for the 7.9 branch.

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