[PM] Refactor how the LoopRotation pass access the DominatorTree.

[PM] Refactor how the LoopRotation pass access the DominatorTree.

Instead of querying the pass every where we need to, do that once and
cache a pointer in the pass object. This is both simpler and I'm about
to add yet another place where we need to dig out that pointer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226391 91177308-0d34-0410-b5e6-96231b3b80d8