Register Allocator Unit Tests

Currently the backend only has end-to-end tests and I think it would benefit from some unit tests starting with (for no special reason) the register allocator.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	Compiler (NCG)
Test case
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