Simplify do-while loops that do not use `continue`.

We can omit the continue mask entirely when we know that `continue`
isn't used. This saves a slot, and two ops per loop iteration.

A similar simplification was applied to for loops at
http://review.skia.org/634359 .

Change-Id: I6c56a189c4cfeb601d9f738678a7dba257c47b74
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/635180
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>