[LOOPINFO] Introduce the loop guard API.

Summary:
This is the first patch for the loop guard. We introduced
getLoopGuardBranch() and isGuarded().
This currently only works on simplified loop, as it requires a preheader
and a latch to identify the guard.
It will work on loops of the form:
/// GuardBB:
///   br cond1, Preheader, ExitSucc <== GuardBranch
/// Preheader:
///   br Header
/// Header:
///  ...
///   br Latch
/// Latch:
///   br cond2, Header, ExitBlock
/// ExitBlock:
///   br ExitSucc
/// ExitSucc:
Prior discussions leading upto the decision to introduce the loop guard
API: http://lists.llvm.org/pipermail/llvm-dev/2019-May/132607.html
Reviewer: reames, kbarton, hfinkel, jdoerfert, Meinersbur, dmgreen
Reviewed By: reames
Subscribers: wuzish, hiraditya, jsji, llvm-commits, bmahjour, etiotto
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D63885

llvm-svn: 367033