Optimize (string= (the string string) "").

Transform into a (zerop (length string)), and transform STRING/= into
(and (plusp (length string)) 0).
Add a comment to x86-64/pred.lisp:move-if.

It can't use XOR to load 0 because that will overwrite the flags.
Optimize (LOGIOR SIGNED UNSIGNED) on x86-64.

Add two VOPs for combinations of unsigned-num and signed-num
arguments.