decode_from_copy should keep its raw bits valid (for non-ctis at least)

Today, decode_from_copy always marks the raw bits invalid, with a comment saying it's avoiding an invalid relative target. But that's mostly a stale worry b/c today encode properly re-relativizes rip-rel and cti-short instrs. It's true that a general cti is not re-relativized there but we could rule that out inside decode_from_copy, or go and implement #731 (closed).