Fix resolveVirtualMethodHelper to check for explicit override cases

Fix resolveVirtualMethodHelper to check for explicit override cases

Explicit method overrides in a class can override a virtual final method
with different method. This is only possible if the different method
first is introduced in a new slot. So when devirtualizing, verify that the
slot of the derived method matches the slot of the base method.

If they don't match, just bail on devirtualizing.

Fixes #19222.