[MachineVerifier] Fix COPY check in MachineVerifier for scalable vectors

This change fixes #71518, which compared the KnownMinValue of the
scalable virtual register with the FixedSize of the physical register in
the wrong direction. It turns out that we cannot include this check at all since
it will lead to a false failures. Test cases are added to show that
the false failures no longer occur after this fix.