glsl_to_tgsi: try to avoid emitting a MOV_SAT to saturate an expression tree
glsl_to_tgsi: try to avoid emitting a MOV_SAT to saturate an expression tree

This is a port of commit 62722d9 to glsl_to_tgsi, with minor aesthetic
changes (moved the declaration and assignment of new_inst inside the if block).

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