[IVDescriptors] Remove getConsecutiveDirection (NFC)
The last use of the function was removed on Sep 18, 2016 in commit
5f8cc0c3469ba3a7aa440b43aaababa3a6274213.
The function was later moved to llvm/lib/Analysis/IVDescriptors.cpp on
Sep 12, 2018 in commit 7e98d69847aefb1028aaa7131b508f4b4e9896ae.