swift234

Perform the RRN optimization early/clear when we see an unknown call.

If we have an unknown call, we need to create any retainN calls we
have seen. The reason why is that we do not want to move retains,
releases over isUniquelyReferenced calls. Specifically imagine this:

retain(x); unknown(x); release(x); isUniquelyReferenced(x); retain(x);

In this case we would with this optimization merge the last retain
with the first. This would then create an additional copy. The
release side of this is:

retain(x); unknown(x); release(x); isUniquelyReferenced(x); release(x);

Again in such a case by merging the first release with the second
release, we would be introducing an additional copy.

Thus if we see an unknown call we merge together all retains and
releases before to get some optimization without any loss of
correctness. This could be made more aggressive through appropriate
alias analysis and usage of LLVM's function attributes to determine that
a function does not touch globals.

Swift SVN r30394