swift234

[+0 self] Teach SILGen how to avoid emitting an extra rr pair on self when calling a dynamic dispatch method on a let.

The main thing that this patch does is work around a shortcoming of
SILGenApply namely that we in certain cases emit self before we know
what the callee is. We work around this by emitting self at +0 assuming
that the callee does pass self at +0 and set a flag. After we know what
the callee is, if the flag is set, we emit an extra retain for self.

rdar://15729033

Swift SVN r27553