enhance collapse_vars (#1862)

- extend expression types
  - `a++`
  - `a=x;`
- extend scan range
  - `for(init;;);`
  - `switch(expr){case expr:}`
  - `a = x; a = a || y;`
- terminate upon `debugger;`

closes #1821
fixes #27
fixes #315
fixes #1858

test/compress/drop-unused.js

@@ -1114,3 +1114,36 @@ issue_1838: {
         "}",
     ]
 }
+
+var_catch_toplevel: {
+    options = {
+        conditionals: true,
+        negate_iife: true,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function f() {
+            a--;
+            try {
+                a++;
+            } catch(a) {
+                if (a) var a;
+                var a = 10;
+            }
+        }
+        f();
+    }
+    expect: {
+        !function() {
+            a--;
+            try {
+                a++;
+            } catch(a) {
+                var a;
+            }
+        }();
+    }
+}