improve unused (#1832)

- extract leading value with side-effects out of `var` statement
- reduce scanning of `AST_Definitions` from 3 passes to just once

test/compress/drop-unused.js

@@ -935,7 +935,8 @@ issue_1715_3: {
             try {
                 console;
             } catch (a) {
-                var a = x();
+                var a;
+                x();
             }
         }
         f();