Remove an unneeded SETF of the result of an INCF in a copy loop.
At least on x86[-64] this shortens the generated code in the byte
basher functions by removing a redundant move instruction.
(flet ((get-next-src ()
(setf prev next)
(setf next (funcall src-ref-fn src
- (setf src-word-offset (incf src-word-offset))))))
+ (incf src-word-offset)))))
(declare (inline get-next-src))
,@(unless (= bytes-per-word 1)
`((unless (zerop dst-byte-offset)