[PBQP] Use a local bit-matrix to speedup searching an edge in the graph.
[PBQP] Use a local bit-matrix to speedup searching an edge in the graph.

Build time (user time) for building llvm+clang+lldb in release mode:
 - default allocator: 9086 seconds
 - with PBQP: 9126 seconds
 - with PBQP + local bit matrix cache: 9097 seconds

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@231360 91177308-0d34-0410-b5e6-96231b3b80d8

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