[GlobalISel][MachineRegisterInfo] Add a method to create generic vregs.

For now, generic virtual registers will not have a register class. We may want
to change that. For instance, if we want to use all the methods from
TargetRegisterInfo with generic virtual registers, we need to either have some
sort of generic register classes that do what we want, or teach those methods
how to deal with nullptr register class.

Although the latter seems easy enough to do, we may still want to differenciate
generic register classes from nullptr to catch cases where nullptr gets
introduced by a bug of some sort.

Anyway, I will file a PR to keep track of that.

llvm-svn: 260474