Fix a silly coding error on my part. The whole point of the speculator

Fix a silly coding error on my part. The whole point of the speculator
being separate was that it can grow the use list. As a consequence, we
can't use the iterator-pair interface, we need an index based interface.
Expose such an interface from the AllocaPartitioning, and use it in the
speculator.

This should at least fix a use-after-free bug found by Duncan, and may
fix some of the other crashers.

I don't have a nice deterministic test case yet, but if I get a good
one, I'll add it.

llvm-svn: 165027