[AArch64] Register (existing) AArch64BranchRelaxation pass with LLVM pass manager.
[AArch64] Register (existing) AArch64BranchRelaxation pass with LLVM pass manager.

Summary: Among other things, this allows -print-after-all/-print-before-all to
dump IR around this pass.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@244060 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm