When making a clone of a DSGraph from the BU pass, make sure to remember that

this clone is supposed to be used for *ALL* of the functions in the SCC.

This fixes the memory explosion problem the TD pass was having, reducing the
memory growth from 24MB -> 3.5MB on povray and 270MB ->8.3MB on perlbmk!
This obviously also speeds up the TD pass *a lot*.

llvm-svn: 20763