[FunctionAttrs] Fix an iterator wraparound bug

[FunctionAttrs] Fix an iterator wraparound bug

Summary:
This change fixes an iterator wraparound bug in
`determinePointerReadAttrs`.

Ideally, ++'ing off the `end()` of an iplist should result in a failed
assert, but currently iplist seems to silently wrap to the head of the
list on `end()++`.  This is why the bad behavior is difficult to
demonstrate.

Reviewers: chandlerc, reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14350

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252386 91177308-0d34-0410-b5e6-96231b3b80d8