This makes make_more_copies do what its documentation says, that is,
only make an intermediate pseudo if copying to a pseudo.
This regressed generated code quality when we didn't keep the original
notes that were on the copy, but since r265582 we do, and only allowing
pseudos now is a win. It also simplifies the code.
* combine.c (make_more_copies): Only make an intermediate copy if the
dest of a move is a pseudo.