RegisterScavenging: Code cleanup; NFC

- Use range based for loops
- No need for some !Reg checks: isPhysicalRegister() reports false for
  NoRegister anyway
- Do not repeat function name in documentation comment.
- Do not repeat documentation comment in implementation when we already
  have one at the declaration.
- Factor some common subexpressions out.
- Change file comments to use doxygen syntax.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274194 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed