Fix use_iterator crash in ObjCArc from r203364

The use_iterator redesign in r203364 introduced an increment past the
end of a range in -objc-arc-contract.  Added an explicit check for the
end of the range.

<rdar://problem/16333235>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204195 91177308-0d34-0410-b5e6-96231b3b80d8