[ObjCArc] Do not dereference an invalidated iterator.

Fix a bug in ARC contract pass where an iterator that pointed to a
deleted instruction was dereferenced.

It appears that tryToContractReleaseIntoStoreStrong was incorrectly
assuming that a call to objc_retain would not immediately follow a call
to objc_release.

rdar://problem/25276306

llvm-svn: 299507