[LFTR] Rename variable to minimize confusion [NFC]

As pointed out by Nikita in D62625, BackedgeTakenCount is generally used to refer to the backedge taken count of the loop.  A conditional backedge taken count - one which only applies if a particular exit is taken - is called a ExitCount in SCEV code, so be consistent here.

llvm-svn: 363293