When Hua Zi 1990 promoted You Are My Legend in various places, it was called the theme song "If there is love in heaven", but after it was not adopted, it was explained in an interview that the film changed its song.
So it can be said that If You Are My Legend is not the theme song of any movie.
(Of course, if you are my legend, there is also a Cantonese version called End of Fate, which is the theme song of the movie "Dragon and Phoenix Teahouse". )