[analyzer] Use a more generic MemRegion.getAsOffset to evaluate bin operators on MemRegions

In addition to enabling more code reuse, this suppresses some false positives by allowing us to
compare an element region to its base. See the ptr-arith.cpp test cases for an example.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182780 91177308-0d34-0410-b5e6-96231b3b80d8