ScopLib: Support negated access functions.

Scoplib only supports access functions, but not the more generic
access relations. This commit now also supports access functions
that where not directly expresses as A[sub] with sub = i + 5b,
but with A[sub] with -sub = -i + (-5b).

Test case to come.

Contributed by: Dustin Feld <d3.feld@gmail.com>

llvm-svn: 165379