enhance booleans (#3661)

test/compress/collapse_vars.js

@@ -5863,8 +5863,8 @@ issue_2974: {
         var c = 0;
         (function(b) {
             var a = 2;
-            for (; b.null = -4, c++, b.null && --a > 0;);
-        })(!0),
+            for (;c++, (!0).null && --a > 0;);
+        })(),
         console.log(c);
     }
     expect_stdout: "1"

test/compress/transform.js

@@ -103,6 +103,7 @@ if_return: {
         booleans: true,
         conditionals: true,
         if_return: true,
+        passes: 2,
         sequences: true,
         side_effects: true,
     }