FIR2IR: Do not add implicit casts for types with different nullability

For smart casts, elvises, etc., there are no implicit casts in psi2fir
in changed test data

compiler/testData/ir/irText/expressions/variableAsFunctionCall.fir.kt.txt

@@ -23,12 +23,12 @@ fun test4(ns: String?): String? {
       val tmp0_safe_receiver: String? = ns
       when {
         EQEQ(arg0 = tmp0_safe_receiver, arg1 = null) -> null
-        else -> tmp0_safe_receiver /*as String */.k()
+        else -> tmp0_safe_receiver.k()
       }
     }
     when {
       EQEQ(arg0 = tmp1_safe_receiver, arg1 = null) -> null
-      else -> tmp1_safe_receiver /*as Function0<String> */.invoke()
+      else -> tmp1_safe_receiver.invoke()
     }
   }
 }