K2: Optimize one-branch when-expr case only for independent context

Using independent one where `when` is nested doesn't look correct

compiler/testData/diagnostics/tests/inference/regressions/kt37419.fir.kt

@@ -1,44 +0,0 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_VARIABLE
-
-interface Receiver
-interface Parameter
-typealias LambdaWithReceiver = Receiver.(Parameter) -> Unit
-
-fun Receiver.method(param: Parameter): LambdaWithReceiver = TODO()
-
-enum class E { VALUE }
-
-fun <K> id(x: K): K = x
-
-class SomeClass {
-    val e = E.VALUE
-
-    val withoutType: LambdaWithReceiver
-        get() = <!RETURN_TYPE_MISMATCH!>when (e) {
-            E.VALUE -> { param ->
-                <!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>method<!>(param)
-            }
-        }<!>
-
-    val withExplicitType: LambdaWithReceiver
-        get() = <!RETURN_TYPE_MISMATCH!>when (e) {
-            E.VALUE -> { param: Parameter ->
-                <!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>method<!>(param)
-            }
-        }<!>
-}
-
-class OtherClass {
-    val ok: LambdaWithReceiver
-        get() = { param: Parameter ->
-            method(param)
-        }
-}
-
-val e2 = E.VALUE
-val staticWithExplicitType: LambdaWithReceiver
-    get() = <!RETURN_TYPE_MISMATCH!>when (e2) {
-        E.VALUE -> { param: Parameter ->
-            <!UNRESOLVED_REFERENCE!>method<!>(param)
-        }
-    }<!>

compiler/testData/diagnostics/tests/inference/regressions/kt37419.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_VARIABLE
 
 interface Receiver