extend fuzzy RHS folding (#3006)

- `a = []; if (1) x();` => `if (a = []) x();`

lib/ast.js

@@ -916,5 +916,25 @@ TreeWalker.prototype = {
                 || node instanceof AST_Break && x instanceof AST_Switch)
                 return x;
         }
+    },
+    in_boolean_context: function() {
+        var self = this.self();
+        for (var i = 0, p; p = this.parent(i); i++) {
+            if (p instanceof AST_SimpleStatement
+                || p instanceof AST_Conditional && p.condition === self
+                || p instanceof AST_DWLoop && p.condition === self
+                || p instanceof AST_For && p.condition === self
+                || p instanceof AST_If && p.condition === self
+                || p instanceof AST_UnaryPrefix && p.operator == "!" && p.expression === self) {
+                return true;
+            }
+            if (p instanceof AST_Binary && (p.operator == "&&" || p.operator == "||")
+                || p instanceof AST_Conditional
+                || p.tail_node() === self) {
+                self = p;
+            } else {
+                return false;
+            }
+        }
     }
 };