[JS Legacy] Fix Unit materialization in overridden methods

^KT-51878 Fixed

js/js.translator/testData/box/coercion/unitMaterializationInOverriddenMethod.kt

@@ -0,0 +1,24 @@
+// EXPECTED_REACHABLE_NODES: 1279
+// KT-51878
+
+open class A {
+    open fun foo(): Any = "string"
+}
+
+var condition = true
+
+class B: A() {
+    override fun foo(): Unit {
+        if (condition) {
+            return
+        }
+        condition = !condition
+    }
+}
+
+fun box(): String {
+    val b: A = B()
+    if (b.foo() !is Unit) return "fail1"
+    if (b.foo() !is Unit) return "fail2"
+    return "OK"
+}