Add support for a top-down propagation pass.

Add support for a top-down propagation pass.
Each DSGraph now keeps a list of pending callers that have not
been inlined into the function represented by that graph.
It also keeps a copy of the original call nodes before the BU pass
eliminates some of them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2965 91177308-0d34-0410-b5e6-96231b3b80d8