IR: Fix scope transparency in ReturnableBlockLowering

An IrReturnableBlock introduces a new variable scope and shouldn't be
replaced with a transparent IrComposite block.

compiler/testData/codegen/box/directInvokeOptimization/kt53202.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    val a = "OK"
+
+    val c = {
+        val b = a
+        b
+    }.invoke()
+
+    return a
+}

compiler/testData/codegen/box/directInvokeOptimization/kt53202_funLiteral.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    val a = "OK"
+
+    val c = (fun(): String {
+        val b = a
+        return b
+    }).invoke()
+
+    return a
+}