Use auto-predication in drreg

#2583 adds auto predication globally, and adds rudimentary support for it for all sample clients. `drreg` requires more work however. Currently `drreg` opts out completely from auto predication, as it doesn't interact well with it (i.e. causes the app to crash).

This issue involves adding support for auto predication to `drreg`, so that `drreg` spills and restores are also predicated as appropriate.

Start by reading the drreg implementation and the changes in issue #2583 that added global auto predication and sample-client support. Trace how drreg currently opts out and how spills and restores are handled; done means drreg supports auto predication without crashes and predicates its spills and restores appropriately.

Written by the indexing model from the issue text.