FIR DFA: align return-implies checker closer to smartcasts

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

@@ -31,9 +31,9 @@ fun testNullWhenNull(x: Int?) {
 
 // NB. it is the same function as `nullWhenNull`, but annotations specifies other facet of the function behaviour
 fun notNullWhenNotNull (x: Int?): Int? {
-    contract {
+    <!WRONG_IMPLIES_CONDITION!>contract {
         returns(null) implies (x == null)
-    }
+    }<!>
     return x?.inc()
 }