K2: Fix false-negative RETURN_TYPE_MISMATCH

^KT-53987 Fixed
^KT-55932 Fixed

compiler/testData/diagnostics/tests/smartCasts/publicVals/delegate.fir.kt

@@ -13,7 +13,7 @@ class Example {
 
     public fun foo(): String {
         // Smart cast is not possible if property is delegated
-        return if (p != null) p else ""
+        return <!RETURN_TYPE_MISMATCH!>if (p != null) p else ""<!>
     }
 
     public fun bar(): String {