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

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

test/compress/issue-1275.js

@@ -35,7 +35,7 @@ string_plus_optimization: {
                 throw "nope";
             }
             try {
-                console.log('0' + throwing_function() ? "yes" : "no");
+                console.log((throwing_function(), "yes"));
             } catch (ex) {
                 console.log(ex);
             }