[scudo] Quarantine optimization

Summary:
It turns out that the previous code construct was not optimizing the allocation
and deallocation of batches. The class id was read as a class member (even
though a precomputed one) and nothing else was optimized. By changing the
construct this way, the compiler actually optimizes most of the allocation and
deallocation away to only work with a single class id, which not only saves some
CPU but also some code footprint.

Reviewers: alekseyshl, dvyukov

Reviewed By: dvyukov

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

Differential Revision: https://reviews.llvm.org/D46961

llvm-svn: 332502