FIR: Choose a resolved candidate for augmented assignment when both

assign and operator candidates are unsuccessful.

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/augmentedAssignment.fir.kt

@@ -1,17 +0,0 @@
-class A {
-    operator fun plusAssign(s: String) {}
-}
-
-fun test() {
-    var a: A? = A()
-    a <!UNSAFE_OPERATOR_CALL!>+=<!> ""
-}
-
-class B {
-    operator fun plus(other: B) = this
-}
-
-fun test2() {
-    var b: B? = B()
-    b <!UNRESOLVED_REFERENCE!>+=<!> B()
-}

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/augmentedAssignment.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class A {
     operator fun plusAssign(s: String) {}
 }