fix corner cases in merge_vars (#5457)

fixes #5456

test/compress/awaits.js

@@ -1748,8 +1748,8 @@ issue_4454_2: {
     expect: {
         function f(a) {
             (async function(c = console.log(a)) {})();
-            var a = 42..toString();
-            console.log(a);
+            var b = 42..toString();
+            console.log(b);
         }
         f("PASS");
     }
@@ -2959,3 +2959,45 @@ issue_5305_3: {
     expect_stdout: "PASS"
     node_version: ">=8"
 }
+
+issue_5456: {
+    options = {
+        inline: true,
+        merge_vars: true,
+    }
+    input: {
+        var a = true;
+        (function() {
+            (function(b, c) {
+                var d = async function() {
+                    c = await null;
+                }();
+                var e = function() {
+                    if (c)
+                        console.log(typeof d);
+                    while (b);
+                }();
+            })(function(i) {
+                return console.log("foo") && i;
+            }(a));
+        })();
+    }
+    expect: {
+        var a = true;
+        (function() {
+            b = (i = a, console.log("foo") && i),
+            i = async function() {
+                c = await null;
+            }(),
+            e = function() {
+                if (c) console.log(typeof i);
+                while (b);
+            }(),
+            void 0;
+            var b, c, i, e;
+            var i;
+        })();
+    }
+    expect_stdout: "foo"
+    node_version: ">=8"
+}