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.
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