Start implementing DSGraph::clonePartiallyInto and implement mergeInGraph

Start implementing DSGraph::clonePartiallyInto and implement mergeInGraph
in terms of it.

Though clonePartiallyInto is not cloning partial graphs yet, this change
dramatically speeds up inlining of graphs with many scalars.  For example,
this change speeds up the BU pass on 253.perlbmk from 69s to 36s, because
it avoids iteration over the scalar map, which can get pretty large.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10951 91177308-0d34-0410-b5e6-96231b3b80d8