MachineDominators: Move applySplitCriticalEdges into the cpp file.

It's too big for inlining anyways. Also clean it up slightly. No functionality
change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230806 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed