Reapply chandlerc's r173342 now that the miscompile it was triggering is fixed.

Original commit message:
Plug TTI into the speculation logic, giving it a real cost interface
that can be specialized by targets.

The goal here is not to be more aggressive, but to just be more accurate
with very obvious cases. There are instructions which are known to be
truly free and which were not being modeled as such in this code -- see
the regression test which is distilled from an inner loop of zlib.

Everywhere the TTI cost model is insufficiently conservative I've added
explicit checks with FIXME comments to go add proper modelling of these
cost factors.

If this causes regressions, the likely solution is to make TTI even more
conservative in its cost estimates, but test cases will help here.