Frontend: Avoid retypechecking the right hand side of assignment operators

compiler/testData/diagnostics/tests/inference/genericAssignmentOperator.kt

@@ -0,0 +1,9 @@
+class R<T>
+
+fun <T> f(): R<T> = R<T>()
+
+operator fun Int.plusAssign(<!UNUSED_PARAMETER!>y<!>: R<Int>) {}
+
+fun box() {
+    1 += f()
+}