[Polly] Fix test case after rL271151

Summary:
After rL271151 (SCEV change) SCEV no longer unconditionally transfers
nuw/nsw from the increment operation to the post-inc value; this
transfer only happens if there is undefined behavior in the program if
the increment overflowed (as opposed to just generating poison).

The loops in `wraping_signed_expr_1.ll` are in non-canonical
form (they're not rotated), and that defeats LLVM's poison-is-UB
analysis.  IMO the easiest fix here is to run `wraping_signed_expr_1.ll`
through `-loop-rotate` to canonicalize the loops, which is what this
patch does.

Reviewers: jdoerfert, Meinersbur, grosser

Subscribers: grosser, mcrosier, pollydev

Differential Revision: http://reviews.llvm.org/D20778

llvm-svn: 271536