[sanitizer] 64-bit allocator's PopulateFreeArray partial refactor

Summary:
This is an attempt at making `PopulateFreeArray` less obscure, more consistent,
and a tiny bit faster in some circumstances:
- use more consistent variable names, that work both for the user & the metadata
  portions of the code; the purpose of the code is mostly the same for both
  regions, so it makes sense that the code should be mostly similar as well;
- replace the while sum loops with a single `RoundUpTo`;
- mask most of the metadata computations behind kMetadataSize, allowing some
  blocks to be completely optimized out if not use metadata;
- `const` the constant variables;
- add a `LIKELY` as the branch it applies to will almost always be taken.

Reviewers: alekseyshl, flowerhack

Reviewed By: alekseyshl

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D40754

llvm-svn: 319673