Update ValueEnumerator.* to be more like external/llvm/.

This change makes ValueEnumerator.cpp and ValueEnumerator.h more like
the copy in external/llvm on which they are already based. This allows us
to make sure that we are not missing any additional functionality (in this
case, the only differences were related to internal debugging dump()

Change-Id: I9a0eb1818fbd6436030482f7c290eb80aee8de8a
4 files changed