Avoid some syscalls in locate-dominating-file

Avoid some syscalls in locate-dominating-file

* lisp/files.el (locate-dominating-file): Do not call
file-directory-p each time through a loop ascending the directory
hierarchy, as the file must be a directory after the first loop
iteration.  Instead, call file-directory-p just once, before the
loop starts, and do this only if the file name is not already that
of a directory.

(cherry picked from commit 2074951c2adff43be49822c344d6cf3fee2e29a5)