improve diagnostics on top_retain & mangle.properties (#5622)

closes #5618

lib/propmangle.js

@@ -260,8 +260,14 @@ function mangle_properties(ast, options) {
     }
 
     function should_mangle(name) {
-        if (reserved.has(name)) return false;
-        if (regex && !regex.test(name)) return false;
+        if (reserved.has(name)) {
+            AST_Node.info("Preserving reserved property {this}", name);
+            return false;
+        }
+        if (regex && !regex.test(name)) {
+            AST_Node.info("Preserving excluded property {this}", name);
+            return false;
+        }
         return cache.has(name) || names_to_mangle.has(name);
     }
 
@@ -271,10 +277,7 @@ function mangle_properties(ast, options) {
     }
 
     function mangle(name) {
-        if (!should_mangle(name)) {
-            AST_Node.info("Preserving property {this}", name);
-            return name;
-        }
+        if (!should_mangle(name)) return name;
         var mangled = cache.get(name);
         if (!mangled) {
             if (debug) {