RegisterCoalescer: Cleanup comment style

- Consistenly put comments above the function declaration, not the
  definition. To achieve this some duplicate comments got merged and
  some comment parts describing implementation details got moved into their
  functions.
- Consistently use doxygen comments above functions.
- Do not use doxygen comments inside functions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226351 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed