[BasicTTI] Return Invalid for scalable vectors reaching getScalarizationOverhead

If we would scalarize a fixed vector, we know we can't do so for a scalable one.  However, there's no need to crash, we can instead simply return a invalid cost which will work its way through the computation (since invalid is sticky), and the client should bail out.

Sorry for the lack of test here.  The particular codepath I saw this reached on was the result of another bug.