Silence GCC 7 warning by using an enum class.

This silences the following GCC7 warning:

    lib/Target/Hexagon/HexagonISelDAGToDAGHVX.cpp:142:30: warning:
    enumeral and non-enumeral type in conditional expression [-Wextra]
         return F != Colors.end() ? F->second : None;
                    ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~

Reviewers: amharc, RKSimon, davide

Reviewed By: RKSimon, davide

Differential Revision: https://reviews.llvm.org/D41003

llvm-svn: 322398