Allow to generate a loop without the GuardBB

  This allows us to omit the GuardBB in front of created loops
  if we can show the loop trip count is at least one. It also
  simplifies the dominance relation inside the new created region.
  A GuardBB (even with a constant branch condition) might trigger
  false dominance errors during function verification.

Differential Revision: http://reviews.llvm.org/D5297