[sanitizer] Allocator local cache improvements

Summary:
Here are a few improvements proposed for the local cache:
- `InitCache` always read from `per_class_[1]` in the fast path. This was not
  ideal as we are working with `per_class_[class_id]`. The latter offers the
  same property we are looking for (eg: `max_count != 0` means initialized),
  so we might as well use it and keep our memory accesses local to the same
  `per_class_` element. So change `InitCache` to take the current `PerClass`
  as an argument. This also makes the fast-path assembly of `Deallocate` a lot
  more compact;
- Change the 32-bit `Refill` & `Drain` functions to mimic their 64-bit
  counterparts, by passing the current `PerClass` as an argument. This saves
  some array computations;
- As far as I can tell, `InitCache` has no place in `Drain`: it's either called
  from `Deallocate` which calls `InitCache`, or from the "upper" `Drain` which
  checks for `c->count` to be greater than 0 (strictly). So remove it there.
- Move the `stats_` updates to after we are done with the `per_class_` accesses
  in an attempt to preserve locality once more;
- Change some `CHECK` to `DCHECK`: I don't think the ones changed belonged in
  the fast path and seemed to be overly cautious failsafes;
- Mark some variables as `const`.

The overall result is cleaner more compact fast path generated code, and some
performance gains with Scudo (and likely other Sanitizers).

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42851

llvm-svn: 324257