[analyzer] Re-apply "Do part of the work to find shortest bug paths up front".
[analyzer] Re-apply "Do part of the work to find shortest bug paths up front".

With the assurance that the trimmed graph does not contain cycles,
this patch is safe (with a few tweaks), and provides the performance
boost it was intended to.

Part of performance work for <rdar://problem/13433687>.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@177469 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang