[Fix] --define replaces SymbolRefs in LHS of assignments

See #208 for context

lib/compress.js

@@ -2277,7 +2277,15 @@ merge(Compressor.prototype, {
     });
 
     OPT(AST_SymbolRef, function(self, compressor){
-        if (self.undeclared()) {
+        function isLHS(symbol, parent) {
+            return (
+                parent instanceof AST_Binary &&
+                parent.operator === '=' &&
+                parent.left === symbol
+            );
+        }
+
+        if (self.undeclared() && !isLHS(self, compressor.parent())) {
             var defines = compressor.option("global_defs");
             if (defines && defines.hasOwnProperty(self.name)) {
                 return make_node_from_constant(compressor, defines[self.name], self);