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/temporaryInInitBlock.kt.txt

@@ -13,7 +13,7 @@ class C {
       val tmp0_safe_receiver: Any? = x
       when {
         EQEQ(arg0 = tmp0_safe_receiver, arg1 = null) -> null
-        else -> tmp0_safe_receiver /*as Any */.toString()
+        else -> tmp0_safe_receiver.toString()
       }
     }
   }