Fix handling of signWrappedSets in access relations

Since r294891, in MemoryAccess::computeBoundsOnAccessRelation(), we skip
manually bounding the access relation in case the parameter of the load
instruction is already a wrapped set. Later on we assume that the lower
bound on the set is always smaller or equal to the upper bound on the
set. Bug 32715 manages to construct a sign wrapped set, in which case
the assertion does not necessarily hold. Fix this by handling a sign
wrapped set similar to a normal wrapped set, that is skipping the
computation.

Contributed-by: Maximilian Falkenstein <falkensm@student.ethz.ch>

Reviewers: grosser

Subscribers: pollydev, llvm-commits

Tags: #Polly

Differential Revision: https://reviews.llvm.org/D32893

llvm-svn: 302231