[PowerPC] Check hot loop exit edge in PPCCTRLoops

PPCCTRLoops transform loops using mtctr/bdnz instructions if loop trip count is known and big enough to compensate for the cost of mtctr.
But if there is a loop exit edge which is known to be frequently taken (by builtin_expect or by PGO), we should not transform the loop to avoid the cost of mtctr instruction. Here is an example of a loop with hot exit edge:

for (unsigned i = 0; i < TripCount; i++) {
  // do something
  if (__builtin_expect(check(), 1))
    break;
  // do something
}

Differential Revision: https://reviews.llvm.org/D42637

llvm-svn: 324229