fix corner case in merge_vars & reduce_vars (#4313)

fixes #4312

test/compress/destructured.js

@@ -1616,3 +1616,34 @@ issue_4308: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_4312: {
+    options = {
+        collapse_vars: true,
+        inline: true,
+        merge_vars: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        (function f(b, c) {
+            return function({
+                [a = b]: d,
+            }) {}(c && c);
+        })("PASS", "FAIL");
+        console.log(a);
+    }
+    expect: {
+        var a;
+        b = "PASS",
+        (function({
+            [a = b]: d,
+        }){})((c = "FAIL") && c);
+        var b, c;
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}

test/compress/functions.js

@@ -521,7 +521,7 @@ issue_2531_2: {
     options = {
         evaluate: true,
         inline: true,
-        passes: 3,
+        passes: 2,
         reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
@@ -556,9 +556,10 @@ issue_2531_3: {
     options = {
         evaluate: true,
         inline: true,
-        passes: 3,
+        passes: 2,
         reduce_funcs: true,
         reduce_vars: true,
+        sequences: true,
         side_effects: true,
         toplevel: true,
         unused: true,