fix corner case in collapse_vars (#5310)

fixes #5309

lib/compress.js

@@ -2119,7 +2119,8 @@ Compressor.prototype.compress = function(node) {
                         var parent = multi_replacer.parent();
                         if (parent instanceof AST_Sequence && parent.tail_node() !== node) {
                             value_def.replaced++;
-                            return List.skip;
+                            if (rvalue === rhs_value) return List.skip;
+                            return make_sequence(rhs_value, rhs_value.expressions.slice(0, -1));
                         }
                         return rvalue;
                       case 1:

test/compress/collapse_vars.js

@@ -9810,3 +9810,38 @@ issue_5277: {
     }
     expect_stdout: "undefined"
 }
+
+issue_5309_1: {
+    options = {
+        collapse_vars: true,
+        conditionals: true,
+        toplevel: true,
+    }
+    input: {
+        if (console)
+            var a = (console.log("PASS"), b), b = a;
+        else
+            console.log("FAIL");
+    }
+    expect: {
+        var a, b;
+        console ? (console.log("PASS"), b = b) : console.log("FAIL");
+    }
+    expect_stdout: "PASS"
+}
+
+issue_5309_2: {
+    options = {
+        collapse_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a, b;
+        console ? (a = (console.log("PASS"), b), b = a) : console.log("FAIL");
+    }
+    expect: {
+        var a, b;
+        console ? (console.log("PASS"), b = b) : console.log("FAIL");
+    }
+    expect_stdout: "PASS"
+}