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/testsWithStdLib/when/kt10807.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 import java.util.*
 import kotlin.comparisons.compareBy
 import kotlin.comparisons.nullsLast
@@ -14,4 +15,4 @@ fun getCompInverted(): Comparator<Foo?> =
                 when {
                     else -> compareBy({ it.a }, { it.b })
                 }
-        )
+        )