r300: Don't disable destination read if the src blend factor needs it
r300: Don't disable destination read if the src blend factor needs it

The read can remain disabled if the src alpha factor needs it because
the result would still be zero.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57984

NOTE: This is a candidate for stable release branches.

Signed-off-by: Marek Olšák <74bac9a67a04041c3586cda013cc5a11352e67be@gmail.com>

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