KT-11724 False suggestion to replace with compound assignment

#KT-11724 Fixed

idea/testData/intentions/replaceWithOperatorAssignment/flexibleTypeBug.kt

@@ -0,0 +1,7 @@
+// IS_APPLICABLE: false
+// WITH_RUNTIME
+fun foo() {
+    var list1 = java.util.Collections.emptyList<String>()
+    val list2 = listOf("b")
+    list1 <caret>= list1 + list2
+}

idea/testData/intentions/replaceWithOperatorAssignment/plusAssignConflict.kt

@@ -0,0 +1,11 @@
+// IS_APPLICABLE: false
+class A
+
+operator fun A.plus(a: A): A = A()
+operator fun A.plusAssign(a: A){}
+
+fun foo() {
+    var a1 = A()
+    val a2 = A()
+    a1 <caret>= a1 + a2
+}