[NFC][InstructionSimplify] Add a warning about not simplifying to not def-reachable

See
https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20200824/824235.html
and
https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20200824/824967.html

InstSimply is not allowed to perform simplifications to instructions
that are not def-reachable from the original instruction.