improve --reduce-test (#3719)

- cover missing cases when eliminating unreferenced labels
- format multi-line outputs correctly

test/input/reduce/unsafe_math.js

@@ -0,0 +1,18 @@
+var _calls_ = 10, a = 100, b = 10, c = 0;
+
+function f0(b_1, a, undefined_2) {
+    a++ + ++b;
+    {
+        var expr2 = (b + 1 - .1 - .1 - .1 || a || 3).toString();
+        L20778: for (var key2 in expr2) {
+            (c = c + 1) + [ --b + b_1, typeof f0 == "function" && --_calls_ >= 0 && f0(--b + typeof (undefined_2 = 1 === 1 ? a : b), --b + {
+                c: (c = c + 1) + null
+            }, a++ + (typeof f0 == "function" && --_calls_ >= 0 && f0(typeof (c = 1 + c, 3 / "a" * ("c" >>> 23..toString()) >= (b_1 && (b_1[(c = c + 1) + a--] = (- -0,
+            true + {})))), 3, 25))), 1 === 1 ? a : b ];
+        }
+    }
+}
+
+var a_1 = f0([ , 0 ].length === 2);
+
+console.log(null, a, b, c, Infinity, NaN, undefined);