[MachineVerifier] Teach how to check some of the properties of generic

virtual registers.

Generic virtual registers:
- May not have a register class
- May not have a register bank
- If they do not have a register class they must have a size
- If they have a register bank, the size of the register bank must be
  greater or equal to the size of the virtual register (basically check
  that the virtual register will fit into that register class)

llvm-svn: 265798