[scudo] Minor Secondary changes
[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

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@323997 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt