[MergeFunc] removeUsers: call remove() only on direct users

removeUsers uses a work list to collect indirect users and call remove()
on those functions. However it has a bug (`if (!Visited.insert(UU).second)`).

Actually, we don't have to collect indirect users.
After the merge of F and G, G's callers will be considered (added to
Deferred). If G's callers can be merged, G's callers' callers will be
considered.

Update the test unnamed-addr-reprocessing.ll to make it clear we can
still merge indirect callers.

llvm-svn: 358741