FIR: extends scope of SENSELESS_COMPARISON

FE1.0 only reports SENSELESS_COMPARISON if one of the operand is `null`.
This change makes FIR reports also in case one of the operand has type
`Nothing?`.

In addition, fix handling of type alias in ConeTypeContext#isNullableType

compiler/testData/diagnostics/tests/when/ElseOnNullableEnumWithSmartCast.fir.kt

@@ -15,6 +15,6 @@ fun foo(e: E, something: Any?): Int {
     return when (e) {
         E.A -> 1
         E.B -> 2
-        something -> 3
+        <!SENSELESS_NULL_IN_WHEN!>something<!> -> 3
     }
 }