[Polly] Remove usage of the `apply` function

Summary:
API-wise `apply` is a somewhat unidiomatic one-off function, and
removing the only(?) use in polly will let me remove it from SCEV's
exposed interface.

Reviewers: jdoerfert, Meinersbur, grosser

Subscribers: grosser, mcrosier, pollydev

Differential Revision: http://reviews.llvm.org/D20779

llvm-svn: 271177