swift234

Add a SIL SCC visitor and an induction variable analysis.

The induction variable analysis derives from the SCC visitor CRTP-style
and uses it to drive analysis to find the IVs of a function.

The current definition of induction variable is very weak, but enough to
use for very basic bounds-check elimination.

This is not quite ready for real use. There is an assert that I've
commented out that is firing but should not be, and that will require
some more investigation.

Swift SVN r19845