r270777 improved the precision of alloca vs. inbounbds GEP alias queries: if
we have (a) an inbounds GEP and (b) a pointer based on an alloca, and the
beginning of the object the GEP points to would have a negative offset with
respect to the alloca, then the GEP can not alias pointer (b).
This makes the same logic fire when (b) is based on a GlobalVariable instead
of an alloca.
Differential Revision: http://reviews.llvm.org/D20652
llvm-svn: 270893