[CodeGen] aritificial subregister should not be generated in tablegen files

This is from this patch https://github.com/llvm/llvm-project/pull/185751 There is a problem found in machine copy propagation pass that a `COPY` is folded into an aritificial subregister which end up with a crash in the following pass.

There seems to be some missing work to systematically recognize forwardable `COPY` of subregisters, and maybe llvm should not emit artificial subregisters into the tablegen files at all.