[LoopInfo] Remove duplicates in ExitBlocks to reduce the compile time of

hasDedicatedExits.

For the compile time problem described in https://reviews.llvm.org/D67359,
turns out the root cause is there are many duplicates in ExitBlocks so
the algorithm complexity of hasDedicatedExits gets very high. If we remove
the duplicates, the compile time issue is gone.

Thanks to Philip Reames for raising a good question and it leads me to
find the root cause.

Differential Revision: https://reviews.llvm.org/D68107

llvm-svn: 373045