fix corner case in collapse_vars (#5644)

fixes #5643

lib/compress.js

@@ -3416,13 +3416,17 @@ Compressor.prototype.compress = function(node) {
                 if (def.references.length - def.replaced == referenced) return true;
                 if (!def.fixed) return false;
                 if (!lhs.fixed) return false;
+                var assigns = lhs.fixed.assigns;
                 var matched = 0;
                 if (!all(def.references, function(ref, index) {
                     var fixed = ref.fixed;
                     if (!fixed) return false;
                     if (fixed.to_binary || fixed.to_prefix) return false;
-                    if (fixed === lhs.fixed) matched++;
+                    if (fixed === lhs.fixed) {
+                        matched++;
                         return true;
+                    }
+                    return assigns && fixed.assigns && assigns[0] !== fixed.assigns[0];
                 })) return false;
                 if (matched != referenced) return false;
                 verify_ref = true;

test/compress/collapse_vars.js

@@ -10098,3 +10098,26 @@ issue_5638_4: {
     }
     expect_stdout: "foo 42"
 }
+
+issue_5643: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = 3, b;
+        a *= 7;
+        b = !!this;
+        console || console.log(b);
+        console.log(a * ++b);
+    }
+    expect: {
+        var a = 3, b;
+        a *= 7;
+        b = !!this;
+        console || console.log(b);
+        console.log(a * ++b);
+    }
+    expect_stdout: "42"
+}