[FIR] Properly support smartcasts on stable when subjects in when conditions

^KT-49860 Fixed

compiler/testData/ir/irText/expressions/when.fir.kt.txt

@@ -15,7 +15,7 @@ fun testWithSubject(x: Any?): String {
       EQEQ(arg0 = tmp0_subject, arg1 = A) -> "A"
       tmp0_subject is String -> "String"
       tmp0_subject !is Number -> "!Number"
-      setOf<Nothing>().contains<Any?>(element = tmp0_subject) -> "nothingness?"
+      setOf<Nothing>().contains<Number>(element = tmp0_subject /*as Number */) -> "nothingness?"
       else -> "something"
     }
   }
@@ -61,3 +61,4 @@ fun testComma(x: Int): String {
     }
   }
 }
+