nir/opt_dead_cf: correction of side effect check
nir/opt_dead_cf: correction of side effect check

Parenthesis are needed here as ! takes precedence over the &. The
check had the opposite effect than intended.

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler