llvmpipe: Fast path for sampling rgba8 textures with linear filtering.
llvmpipe: Fast path for sampling rgba8 textures with linear filtering.

Implement Keith's suggestion of doing most of the sampling with 16x8
and 8x16 AoS, and only doing the conversion to floating point SoA at
the very last step.

Improves gloss performance by 10%.

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