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

^KT-49860 Fixed

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/CFGNode.kt

@@ -794,6 +794,12 @@ class ElvisExitNode(owner: ControlFlowGraph, override val fir: FirElvisExpressio
     }
 }
 
+class WhenSubjectExpressionExitNode(owner: ControlFlowGraph, override val fir: FirWhenSubjectExpression, level: Int, id: Int) : CFGNode<FirWhenSubjectExpression>(owner, level, id) {
+    override fun <R, D> accept(visitor: ControlFlowGraphVisitor<R, D>, data: D): R {
+        return visitor.visitWhenSubjectExpressionExitNode(this, data)
+    }
+}
+
 // ----------------------------------- Other -----------------------------------
 
 @OptIn(CfgInternals::class)

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/CFGNodeRenderer.kt

@@ -99,6 +99,8 @@ fun CFGNode<*>.render(): String =
                 is EnterSafeCallNode -> "Enter safe call"
                 is ExitSafeCallNode -> "Exit safe call"
 
+                is WhenSubjectExpressionExitNode -> "Exit ${'$'}subj"
+
                 is PostponedLambdaEnterNode -> "Postponed enter to lambda"
                 is PostponedLambdaExitNode -> "Postponed exit from lambda"
 

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/ControlFlowGraphVisitor.kt

@@ -347,6 +347,10 @@ abstract class ControlFlowGraphVisitor<out R, in D> {
         return visitNode(node, data)
     }
 
+    open fun visitWhenSubjectExpressionExitNode(node: WhenSubjectExpressionExitNode, data: D): R {
+        return visitNode(node, data)
+    }
+
     // ----------------------------------- Elvis -----------------------------------
 
     open fun visitElvisLhsExitNode(node: ElvisLhsExitNode, data: D): R {