vc4: Enable dead CF elimination.

Now that we're about to start generating control flow in our NIR, we want
this in place.  It optimizes things frequently in the CS, when the GL VS
has control flow that doesn't affect the vertex position.

src/gallium/drivers/vc4/vc4_program.c

@@ -1337,6 +1337,7 @@ vc4_optimize_nir(struct nir_shader *s)
 
                 NIR_PASS(progress, s, nir_copy_prop);
                 NIR_PASS(progress, s, nir_opt_dce);
+                NIR_PASS(progress, s, nir_opt_dead_cf);
                 NIR_PASS(progress, s, nir_opt_cse);
                 NIR_PASS(progress, s, nir_opt_peephole_select);
                 NIR_PASS(progress, s, nir_opt_algebraic);