FIR: Fix resolving += to ignore + if it doesn't return correct type

This is the FIR fix of the FE change tracked by
https://youtrack.jetbrains.com/issue/KT-45503

Consider the following code:

```
fun foo() {
  var l = mutableListOf("")
  l += ""
}
```

The above code used to be considered invalid due to ambiguous `+=`. But
after KT-45503, it's now considered valid code because `plus` is not
really a valid option for this code. This is because the return type
of `plus` for `List` is `List`, which does not match the type of
variable `l`.

As for now, FIR rejects the above code due to ambiguous `+=`. This
change fixes that by also consider the return type when resolving `+=`.

compiler/testData/diagnostics/tests/operatorsOverloading/plusAssignOnVarAndCollections.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // WITH_RUNTIME
 
 fun test1() {