SIL: Add SuperToArchetypeInst.

Add an instruction to represent super-to-archetype downcasts. Since archetypes are address-only, it needs to take a reference to the destination archetype as a parameter.

Swift SVN r3643

lib/SIL/SILPrinter.cpp

@@ -274,6 +274,11 @@ public:
   void visitArchetypeToSuperInst(ArchetypeToSuperInst *CI) {
     printConversionInst(CI, "archetype_to_super");
   }
+
+  void visitSuperToArchetypeInst(SuperToArchetypeInst *I) {
+    OS << "super_to_archetype " << getID(I->getSrcBase()) << " to "
+       << getID(I->getDestArchetypeAddress());
+  }
   
   void visitTupleInst(TupleInst *TI) {
     OS << "tuple (";