ADT::GenericCycleInfo: Hide validateTree() in -Asserts.

validateTree() is instantiated with __FILE__.
It will be pruned at link time due to -ffunction-sections but be left in
object files.
Its user is only GenericCycleInfo::compute() with assert(validateTree());
Therefore I think validateTree() may be hidden with NDEBUG.

This is a fixup for https://reviews.llvm.org/D112696