i965/fs: Avoid inappropriate optimization with regs_written > 1.
i965/fs: Avoid inappropriate optimization with regs_written > 1.

Right now we don't have anything with regs_written() > 1 and !inst->mlen,
but that's about to change.

NOTE: This is a candidate for the 9.1 branch.
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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