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.

llvm-svn: 274194