i#4134 drbbdup: Use spill slot enum

[derekbruening]

Changes hardcoded slot numbers to use the corresponding symbolic enum
value instead.

Re-orders a too-late dead register analysis.

Fixes a typo in the drbbdup docs.

Issue: #4134

[johnfxgalea]

Nit: Maybe change drbbdup_spill_register() and drbbdup_restore_register() to take type drbbdup_thread_slots_t, instead of int, for index passed as param.

[derekbruening]

Nit: Maybe change drbbdup_spill_register() and drbbdup_restore_register() to take type drbbdup_thread_slots_t, instead of int, for index passed as param.

Done.