Add TODO comment for the decreasing induction

Author: Mel-Chen

llvm/lib/Analysis/IVDescriptors.cpp

@@ -742,6 +742,7 @@ RecurrenceDescriptor::isFindLastIVPattern(PHINode *OrigPhi, Instruction *I,
   // We are looking for selects of the form:
   //   select(cmp(), phi, loop_induction) or
   //   select(cmp(), loop_induction, phi)
+  // TODO: Support for monotonically decreasing induction variable
   if (!IsIncreasingLoopInduction(NonRdxPhi))
     return InstDesc(false, I);