The InductionDescriptor and RecurrenceDescriptor classes basically analyzes the IR to identify the respective IVs. So, it is better to have them in the "Analysis" directory instead of the "Transforms" directory.
The rationale for this is to make the Induction and Recurrence descriptor classes available for analysis passes. Currently including them in an analysis pass produces link error (http://lists.llvm.org/pipermail/llvm-dev/2018-July/124456.html).
Base classes for the above classes are in the new file - Analysis/LoopAnalysisUtils.h. This file is included in Transforms/LoopUtils.h. Respective definitions reside in Analysis/LoopAnalysisUtils.cpp and Transforms/LoopUtils.cpp.