FIR DFA: update receivers properly in case of single flow in merge point

compiler/testData/codegen/box/fir/LookupTags.kt

@@ -0,0 +1,39 @@
+// TARGET_BACKEND: JVM
+
+abstract class ConeClassLikeLookupTag {
+    abstract val status: String
+}
+
+class ConeClassLikeLookupTagImpl : ConeClassLikeLookupTag() {
+    override var status: String = ""
+        private set
+
+    fun change(newStatus: String) {
+        status = newStatus
+    }
+}
+
+class ConeClassLikeErrorLookupTag : ConeClassLikeLookupTag() {
+    override val status: String
+        get() = "ERROR"
+}
+
+fun ConeClassLikeLookupTag.foo(): String {
+    (this as? ConeClassLikeLookupTagImpl)?.status?.takeIf { it == "OK" }?.let { return it }
+    return status.also {
+        (this as? ConeClassLikeLookupTagImpl)?.bar(it)
+    }
+}
+
+fun ConeClassLikeLookupTagImpl.bar(s: String) {
+    change(s)
+}
+
+fun box(): String {
+    val tag = ConeClassLikeErrorLookupTag()
+    tag.foo()
+    val tag2 = ConeClassLikeLookupTagImpl()
+    tag2.change("OK")
+    tag2.foo()
+    return tag2.status
+}