i965: make swizzle_for_size() return unsigned
i965: make swizzle_for_size() return unsigned

Silences a warning about comparing to an unsigned variable.  It looks like
the result of swizzle_for_size() is always assigned to unsigned vars.

Reviewed-by: Chad Versace <c34c009102e60d77975598e1f414f8a701c22d0b@chad-versace.us>

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