vc4: DCE instructions with a NULL destination.
vc4: DCE instructions with a NULL destination.

I'm going to add an optimization for redundant SF update removal, which
will just remove the SF and leave us (in many cases) with an instruction
with a NULL destination and no side effects.  Rather than teaching that
pass whether the whole instruction can be removed, leave that
responsibility to this pass.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler