bpf: refactor propagate_liveness to eliminate duplicated for loop

Propagation for register and stack slot are finished in separate for loop,
while they are perfect to be put into a single loop.

This could also let them share some common variables in later patches.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -6254,10 +6254,8 @@ static int propagate_liveness(struct bpf_verifier_env *env,
 					return err;
 			}
 		}
-	}
 
-	/* ... and stack slots */
+		/* Propagate stack slots. */
-	for (frame = 0; frame <= vstate->curframe; frame++) {
 		state = vstate->frame[frame];
 		parent = vparent->frame[frame];
 		for (i = 0; i < state->allocated_stack / BPF_REG_SIZE &&