[scudo] Minor Secondary changes

Summary:
Few changes to the secondary:
- mark `const` variables as such;
- change some `CHECK` to `DCHECK`: I don't feel we need to be as conservative as
  we were with out checks, as they are the results of our own computation.
- mark a condition as `UNLIKELY`.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42696

llvm-svn: 323997