[RewriteStatepointsForGC] Fix another order of iteration bug

It turns out the naming of inserted phis and selects is sensative to the order in which two sets are iterated.  We need to nail this down to avoid non-deterministic output and possible test failures.  

The modified test is the one I first noticed something odd in.  The change is making it more strict to report the error.  With the test change, but without the code change, the test fails roughly 1 in 5.  With the code change, I've run ~30 runs without error.

Long term, the right fix here is to adjust the naming scheme.  I'm checking in this hack to avoid any possible non-determinism in the tests over the weekend.  HJust because I only noticed one case doesn't mean it's actually the only case.  I hope to get to the right change Monday.

std->llvm data structure changes bugfix change #3

llvm-svn: 230835