Allocation stack lowering based on escape analysis

Now that we can run libgo benchmarks:
https://codereview.appspot.com/103550047/
we can measure the effect of optimizations to see if they are worth adding
to the compiler.

This introduces an ssaopt package to hold SSA-level optimizations, to which
we add an allocation stack lowering pass based on a simple intraprocedural
escape analysis. In an experiment comparing libgo benchmark performance
before and after this change, this pass yielded a geo-mean 7.1% (5.5-8.7%
at a 95% confidence interval) improvement in performance.
Independent of the interprocedurality issue, I would imagine that any analysis which takes into account (i.e. does not simply pessimize, as we currently do) stores of potentially-escaping pointers would also need to take into account the possibility that such a store may escape into a future loop iteration.