gallivm: only use lp_build_conv 4x4f -> 1x16 ub fastpath with sse2
gallivm: only use lp_build_conv 4x4f -> 1x16 ub fastpath with sse2

This is relying on lp_build_pack2 using the sse2 pack intrinsics which
handle clamping.
(Alternatively could have make it use lp_build_packs2 but it might
not even produce more efficient code than not using the fastpath
in the first place.)

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