[XRay] Lazily compute MachineLoopInfo instead of requiring it.

Summary:
Currently X-Ray Instrumentation pass has a dependency on MachineLoopInfo
(and thus on MachineDominatorTree as well) and we have to compute them
even if X-Ray is not used. This patch changes it to a lazy computation
to save compile time by avoiding these redundant computations.

Reviewers: dberris, kubamracek

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D44666

llvm-svn: 327999