glsl: Refactor logic for determining whether to lower return statements.
glsl: Refactor logic for determining whether to lower return statements.

Previously, do_lower_jumps.cpp determined whether to lower return
statements in ir_lower_jumps_visitor::should_lower_jumps().  Moved
this logic to ir_lower_jumps_visitor::visit(ir_function_signature *),
so that it can be used in determining whether to lower a return
statement at the end of a function.

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