Rewrite gtIsVtableAccess to avoid assert

Rewrite gtIsVtableAccess to avoid assert

Calling `HasIndex` from `gtIsVtableAccess` can leads to asserts as the
underly code checks to see if the addressing sub-expressions are
contained. But earlyProp runs early enough that it is not concerned with
containment. The code just needs to verify that there is a ref type base
and no index. Rework the code to do the checks directly.

Commit migrated from https://github.com/dotnet/coreclr/commit/43e5f18ff826058a4de71491a378e5b94ecec919