Give every leaf register a unique regunit, even if it has ad hoc
aliases.
Previously only leaf registers *without* ad hoc aliases would get a
unique regunit, but that caused situations where regunits could not be
used to distinguish a register from its subregs. For example:
- Registers A and B alias. They both get regunit 0 only.
- Register C has subregs A and B. It inherits regunits from its subregs,
so it also gets regunit 0 only.
After this fix, registers A and B will get a unique regunit in addition
to the regunit representing the alias, for example:
- A will get regunits 0 and 1.
- B will get regunits 0 and 2.
- C will get regunits 0, 1 and 2.
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