[LoopInfo] Port loop exit interfaces from Loop to LoopBase

This patch ports hasDedicatedExits, getUniqueExitBlocks and
getUniqueExitBlock in Loop to LoopBase so that they can be used
from other LoopBase sub-classes.

Reviewers: chandlerc, sanjoy, hfinkel, fhahn

Reviewed By: chandlerc

Differential Revision: https://reviews.llvm.org/D48817

llvm-svn: 336572