swift234

Rework the assign instruction in two major ways:

1. Change assign to follow SILGen's model for a semantic assignment,
   which (in the case of unowned pointers) doesn't require the left
   and right hand sides to be the same.
2. Change assign to consume/take the refcount from the LHS, following
   SILGen's design (which I completely misunderstood before).



Swift SVN r7787