FIR CFG: join/unify data flow from postponed lambdas at each level

For example:

    foo(
        // `if` joins A & B
        if (condition)
            run { ... } // A
        else
            run { ... }, // B
        run { ... } // C
    ) // `foo` unifies `A & B` and `C`, so if it is not resolved itself,
      // further `if`s, `when`s, safe calls outside it, etc. continue
      // building the correct type predicate until the next completed
      // call.

^KT-44512 Fixed

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

@@ -228,17 +228,25 @@ class PostponedLambdaEnterNode(owner: ControlFlowGraph, override val fir: FirAno
         return visitor.visitPostponedLambdaEnterNode(this, data)
     }
 }
+
 class PostponedLambdaExitNode(owner: ControlFlowGraph, override val fir: FirAnonymousFunctionExpression, level: Int, id: Int) : CFGNode<FirAnonymousFunctionExpression>(owner, level, id) {
     override fun <R, D> accept(visitor: ControlFlowGraphVisitor<R, D>, data: D): R {
         return visitor.visitPostponedLambdaExitNode(this, data)
     }
 }
+
 class UnionFunctionCallArgumentsNode(owner: ControlFlowGraph, override val fir: FirElement, level: Int, id: Int) : CFGNode<FirElement>(owner, level, id) {
     override fun <R, D> accept(visitor: ControlFlowGraphVisitor<R, D>, data: D): R {
         return visitor.visitUnionFunctionCallArgumentsNode(this, data)
     }
 }
 
+class MergePostponedLambdaExitsNode(owner: ControlFlowGraph, override val fir: FirElement, level: Int, id: Int) : CFGNode<FirElement>(owner, level, id) {
+    override fun <R, D> accept(visitor: ControlFlowGraphVisitor<R, D>, data: D): R {
+        return visitor.visitMergePostponedLambdaExitsNode(this, data)
+    }
+}
+
 class AnonymousFunctionExpressionExitNode(owner: ControlFlowGraph, override val fir: FirAnonymousFunctionExpression, level: Int, id: Int) : CFGNode<FirAnonymousFunctionExpression>(owner, level, id) {
     override fun <R, D> accept(visitor: ControlFlowGraphVisitor<R, D>, data: D): R {
         return visitor.visitAnonymousFunctionExpressionExitNode(this, data)

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

@@ -107,6 +107,7 @@ fun CFGNode<*>.render(): String =
                 is AnonymousFunctionExpressionExitNode -> "Exit anonymous function expression"
 
                 is UnionFunctionCallArgumentsNode -> "Call arguments union"
+                is MergePostponedLambdaExitsNode -> "Merge postponed lambda exits"
 
                 is ClassEnterNode -> "Enter class ${owner.name}"
                 is ClassExitNode -> "Exit class ${owner.name}"

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

@@ -46,6 +46,10 @@ abstract class ControlFlowGraphVisitor<out R, in D> {
         return visitNode(node, data)
     }
 
+    open fun visitMergePostponedLambdaExitsNode(node: MergePostponedLambdaExitsNode, data: D): R {
+        return visitNode(node, data)
+    }
+
     open fun visitAnonymousFunctionExpressionExitNode(node: AnonymousFunctionExpressionExitNode, data: D): R {
         return visitNode(node, data)
     }

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

@@ -32,6 +32,10 @@ abstract class ControlFlowGraphVisitorVoid : ControlFlowGraphVisitor<Unit, Nothi
         visitNode(node)
     }
 
+    open fun visitMergePostponedLambdaExitsNode(node: MergePostponedLambdaExitsNode) {
+        visitNode(node)
+    }
+
     // ----------------------------------- Anonymous object -----------------------------------
 
     open fun visitAnonymousObjectExitNode(node: AnonymousObjectExitNode) {
@@ -325,6 +329,10 @@ abstract class ControlFlowGraphVisitorVoid : ControlFlowGraphVisitor<Unit, Nothi
         visitUnionFunctionCallArgumentsNode(node)
     }
 
+    final override fun visitMergePostponedLambdaExitsNode(node: MergePostponedLambdaExitsNode, data: Nothing?) {
+        visitMergePostponedLambdaExitsNode(node)
+    }
+
     // ----------------------------------- Anonymous object -----------------------------------
 
     final override fun visitAnonymousObjectExitNode(node: AnonymousObjectExitNode, data: Nothing?) {