swift234

"Fix" a use-after-free when dealing with dead cleanups, where we try
to look at the first element of an empty stack. The use-after-free
triggers occasionally for me during normal testing (about 1 failure
per 5 trials), but triggers on IRgen/func.swift regularly when using
guardmalloc. 

John, please take a look; this feels more like treating a symptom than
addressing the cause.




Swift SVN r1389