auto-tune nanobench in all builds

I'm kind of getting bored of having to pass --loops 0
in optimized+asserts builds.

We'd been defaulting to 1 loop in Debug and ASAN builds,
and manually setting Valgrind builds to 1 loop with 1
sample.  Remove the default so all builds auto-tune,
but set Debug and ASAN bots like Valgrind bots.

Change-Id: Ifedd98a11ea74a2c222fe59bd50b96683d12b98c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/224816
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
7 files changed