swift234

DI changes in preparation for conditional 'self consumed' logic

Factor out updateControlVariable() and testControlVariable(), and
split up the release lowering code into multiple methods.

For early returns from designated inits, we can end up with blocks
where one incoming edge has a partially-initialized self, and another
edge has a consumed self. To handle this, we need to emit a test
for the consumed self where only the false branch does the remainder
of the conditional destroy.

This is also a step towards eliminating the null_class instruction.

NFC for now.

Swift SVN r32542