[LoopInfo] Update getExitEdges to accept vector of pairs for non const BasicBlock

D63921 requires getExitEdges fills a vector of Edge pairs where
BasicBlocks are not constant.

The rest Loop API mostly returns non-const BasicBlocks, so to be more consistent with
other Loop API getExitEdges is modified to return non-const BasicBlocks as well.

This is an alternative solution to D64060. 

Reviewers: reames, fhahn
Reviewed By: reames, fhahn
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D64309

llvm-svn: 365437