gallivm: Re-add the kludge for lp_build_lerp of fixed point types.
gallivm: Re-add the kludge for lp_build_lerp of fixed point types.

I removed it in commit 7d44d354bdba853e453ce3991396e2b0933468f4 but
texture sample code still relies on it.

Not sure how to this cleanly, so put it pack for now.

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