Introduce FIR_IDENTICAL for FIR vs old frontend tests #KT-36879 Fixed

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/safecallAndReturnsNull.fir.kt

@@ -1,17 +0,0 @@
-// !LANGUAGE: +AllowContractsForCustomFunctions +UseReturnsEffect
-// !USE_EXPERIMENTAL: kotlin.contracts.ExperimentalContracts
-
-import kotlin.contracts.*
-
-fun Any.nullWhenString(): Any? {
-    contract {
-        returns(null) implies (this@nullWhenString is String)
-    }
-    return if (this is String) null else this
-}
-
-fun test(x: Int?) {
-    if (x?.nullWhenString() == null) {
-        x.<!UNRESOLVED_REFERENCE!>length<!>
-    }
-}

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/safecallAndReturnsNull.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !LANGUAGE: +AllowContractsForCustomFunctions +UseReturnsEffect
 // !USE_EXPERIMENTAL: kotlin.contracts.ExperimentalContracts