Remove complexity in findBoundariesInBlock
We can remove a ton of complexity by assuming that SSA uses never
occur before a def in the same block. This will hold true as long as
useless phis are only removed after all unreachable code is first
removed. Then we don't have the self-loop problem.
The SILVerifier already checks this invariant and I added an in-depth
comment in SimplifyCFG.
