Restore real bit lengths on PPC register numbers
Restore real bit lengths on PPC register numbers

As suggested by Bill Schmidt (in reviewing r178067), use the real register
number bit lengths (which is self-documenting, and prevents using illegal
numbers), and set only the relevant bits in HWEncoding (which defaults to 0).

No functionality change intended.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@178077 91177308-0d34-0410-b5e6-96231b3b80d8

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