[NI] Improve inference for lambdas that return lambdas

compiler/testData/diagnostics/tests/inference/constraints/returnLambdaFromLambda.kt

@@ -0,0 +1,22 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+fun testLambda() {
+    val basicTest: (Int) -> Int = myRun {
+        val x: Any? = null
+        if (x is String) return@myRun { it -> <!DEBUG_INFO_SMARTCAST!>x<!>.length + it }
+        if (x !is Int) return@myRun { it -> it }
+
+        { it -> <!DEBUG_INFO_SMARTCAST!>x<!> + it }
+    }
+
+    val twoLambda: (Int) -> Int = myRun {
+        val x: Int = 1
+        run {
+            val y: Int = 2
+            { x + y }
+        }
+    }
+
+}
+
+inline fun <R> myRun(block: () -> R): R = block()

compiler/testData/diagnostics/tests/inference/constraints/returnLambdaFromLambda.txt

@@ -0,0 +1,4 @@
+package
+
+public inline fun </*0*/ R> myRun(/*0*/ block: () -> R): R
+public fun testLambda(): kotlin.Unit