glsl/linker: Free any IR discarded by optimization passes.
glsl/linker: Free any IR discarded by optimization passes.

Previously, IR for a linked shader was allocated directly out of the
gl_shader object - meaning all of it lived as long as the shader.

Now, IR is allocated out of a temporary context, and any -live- IR is
reparented/stolen to (effectively) the gl_shader.  Any remaining IR can
be freed.

NOTE: This is a candidate for the 7.9 branch.

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