Some added analysis to a discussion which began here.
Seemingly, the understanding of Rashi that Jochebed was one of the seventy souls of Jacob's family who immigrated to Egypt is only explicable according to commentary of the Rabbi Avraham Ibn Ezra (1092-1167) who explained that only the Egyptians of the previous generation died out before the Jews' enslavement began. This is because if the entire generation of Jews died before the enslavement, then Jochebed should have died before the enslavement, and she therefore could not have been the heroine of the story of the Exodus, so if Jochebed were to be one of those who descended to Egypt, Rashi had to understand that only the Egyptians of that generation died before the enslavement, but not necessarily the Jews. However, if this is true, then Rashi did not have to explain that Jochebed was born between the walls of no man's land between the Egyptian borders, explaining that she was born before the Jacobean family descended to Egypt at all would have sufficed. Rather, one can explain that Rashi could even have understood like the Chizkuni or like his grandson that all the Jews of Joseph's generation died before the slavery began, but Jochebed was not considered part of that generation specifically because she was born "between the walls". Therefore, she was considered close enough to the previous generation to merit being enumerated in the seventy souls who immigrated to Egypt, yet she did not die before the indenturing of the Israelites as slaves because she was considered part of the next generation. Consequently, Rashi is not restricted to the explanation of Ibn Ezra.
 To Genesis 46:15
 To Exodus 1:6
 I arrived to this conclusion with the help of Rabbi Michael Katz on the last day of Passover, 5767
 Incidentally, the words of Ibn Ezra himself (to Exodus 1:8) require further examination because, in explaining the verse concerning the "new king" he ignored the both explanations of Rav and of Shmuel from the Midrash and Talmud, and he explains that "new king" refer to the establishment of a new dynasty starting from a king who had no royal blood within him.