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

compiler/testData/diagnostics/tests/smartCasts/variables/whileTrue.fir.kt

@@ -1,13 +0,0 @@
-fun x(): Boolean { return true }
-
-public fun foo(pp: Any): Int {
-    var p = pp
-    while(true) {
-        (p as String).length
-        if (x()) break
-        p = 42
-    }
-    // Smart cast is NOT possible here
-    // (we could provide it but p = 42 makes it difficult to understand)
-    return p.<!UNRESOLVED_REFERENCE!>length<!>()
-}