fold cascade functionality into collapse_vars (#2586)

test/compress/drop-unused.js

@@ -685,7 +685,7 @@ drop_value: {
 
 issue_1539: {
     options = {
-        cascade: true,
+        collapse_vars: true,
         sequences: true,
         side_effects: true,
         unused: true,
@@ -732,7 +732,7 @@ vardef_value: {
 
 assign_binding: {
     options = {
-        cascade: true,
+        collapse_vars: true,
         side_effects: true,
         unused: true,
     }
@@ -1273,7 +1273,7 @@ issue_2226_1: {
 
 issue_2226_2: {
     options = {
-        cascade: true,
+        collapse_vars: true,
         sequences: true,
         side_effects: true,
         unused: true,
@@ -1286,8 +1286,8 @@ issue_2226_2: {
     }
     expect: {
         console.log(function(a, b) {
-            return a += b;
-        }(1, 2));
+            return a += 2;
+        }(1));
     }
     expect_stdout: "3"
 }