(1) Rematerialize nodes from the globals graph into the current graph

(1) Rematerialize nodes from the globals graph into the current graph
    after all callers are inlined into the current graph.
(2) Optimize the way a graph is inlined into its callees in the TD phase:
    (a) Use DSGraph::cloneReachableSubgraph to clone only a subgraph at
        each call site, for faster inlining.
    (b) Clone separately for the same callee at different call sites,
        since only the reachable subgraph is being cloned, not the entire
        caller graph.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7188 91177308-0d34-0410-b5e6-96231b3b80d8