optimize try-catch-finally (#1731)

- eliminate empty blocks
- flatten out if try-block does not throw

test/compress/issue-1673.js

@@ -70,6 +70,7 @@ side_effects_finally: {
         function f() {
             function g() {
                 try {
+                    x();
                 } catch (e) {
                 } finally {
                     console.log("PASS");
@@ -83,6 +84,7 @@ side_effects_finally: {
         function f() {
             (function() {
                 try {
+                    x();
                 } catch (e) {
                 } finally {
                     console.log("PASS");