[BasicAA] Extend inbound GEP negative offset logic to GlobalVariables

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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270893 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed