[MachineRegisterInfo] Track register bank for virtual registers.

A virtual register may have either a register bank or a register class.
This is represented by a PointerUnion between the related classes.

Typically, a virtual register went through the following states
regarding register class and register bank:

1. Creation: None is set. Virtual registers are fully generic.
2. Register bank assignment: Register bank is set. Virtual registers
live into a register bank, but we do not know the constraints they need
to fulfil.
3. Instruction selection: Register class is set. Virtual registers are
bound by encoding constraints.

To map these states to GlobalISel, the IRTranslator implements #1,
RegBankSelect #2, and Select #3.

llvm-svn: 265696