[TailDuplicator] Don't constrain register classes due to debug instructions

If cloning a DBG_VALUE instruction, register uses in that instruction could
lead to constraining of a virtual register that would not happen if the
DBG_VALUE was not present at all. This lead to different code with/without
debug info.

Now we only do that register class constraining if we dealing with a non
debug instruction.

Differential Revision: https://reviews.llvm.org/D149146