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

include/swift/SIL/SILInstruction.h

@@ -616,6 +616,10 @@ public:
   SILValue getSrc() const { return Operands[Src].get(); }
   SILValue getDest() const { return Operands[Dest].get(); }
 
+  bool isUnownedAssign() const {
+    return getDest().getType().getObjectType().is<UnownedStorageType>();
+  }
+
   ArrayRef<Operand> getAllOperands() const { return Operands.asArray(); }
   MutableArrayRef<Operand> getAllOperands() { return Operands.asArray(); }