GlobalISel: verify that a COPY is trivial when created.

Without this check, COPY instructions can actually be one of the generic casts
in disguise. That's confusing and bad.

At some point during ISel this restriction has to be relaxed since the fully
selected instructions will usually use COPY for those purposes. Right now I
think it's possible that relaxation occurs during RegBankSelect (hence the
change there). I'm not convinced that's where it belongs long-term though.

llvm-svn: 306470