Fix DebugInfo replaceAllUsesWith.

Summary:
replaceAllUsesWith had been modified to allow a DbgNode value to be
replaced by itself. In that case a new node is created by copying the
current DbgNode and the copy is used as replacement value.

When that copying happens, the value stored in this->DbgNode at the end
of RAUW would be a reference to the Node that has just been deleted.

This doesn't produce any bug right now, because the DI node on which we
call RAUW won't be used again.

Reviewers: dblaikie, echristo, aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5326

llvm-svn: 217749