swift234

Rewrite switch dispatch emission.

Specialization now recurses on only that prefix of the
current matrix which shares a specialization form
(essentially, a pattern kind) with the head row.  This is
inferior to the previous algorithm in a number of ways: we
may require more switches to perform a single dispatch, and
we may introduce more redundant variables in the leaves.
However, it also means that we will have fully specialized a
row along exactly one path in the decision tree, which makes
it much easier to work with dispatches that introduce new
cleanups.

This change also changes switch-dispatch to use the new
dynamic-cast instructions.

Incidentally fixes rdar://16401831.

Swift SVN r19336