ir_to_mesa: Remove the "ir_to_mesa_" prefix on src_reg/dst_reg types.
ir_to_mesa: Remove the "ir_to_mesa_" prefix on src_reg/dst_reg types.

Both classes are completely private to ir_to_mesa.cpp, so there won't be
any name conflicts with other parts of Mesa.  The prefix simply makes it
harder to read.

Also, use a class rather than typedef structs.

Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

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