warn & drop #__PURE__ iff IIFE is dropped (#1511)

- consolidate `side-effects` optimisations
- improve string `+` optimisation
- enhance literal & `conditionals` optimisations

test/mocha/minify.js

@@ -154,6 +154,17 @@ describe("minify", function() {
             var code = result.code;
             assert.strictEqual(code, "//  comment1   comment2\nbar();");
         });
+        it("should not drop #__PURE__ hint if function is retained", function() {
+            var result = Uglify.minify("var a = /*#__PURE__*/(function(){return 1})();", {
+                fromString: true,
+                output: {
+                    comments: "all",
+                    beautify: false,
+                }
+            });
+            var code = result.code;
+            assert.strictEqual(code, "var a=/*#__PURE__*/function(){return 1}();");
+        })
     });
 
     describe("JS_Parse_Error", function() {