[opened-archetype] When registering values, do not assert on seeing the same definition twice.

Do assert though if we see a different definition.

This allows for one to use the same opened archetype tracker to gather dependent
types from multiple instructions safely. Previously, any overlap would result in
this assert firing.

rdar://31521023

Author: gottesmm

lib/SIL/SILOpenedArchetypesTracker.cpp

@@ -28,7 +28,7 @@ void SILOpenedArchetypesTracker::addOpenedArchetypeDef(CanArchetypeType archetyp
       OldDef = SILValue();
     }
   }
-  assert(!OldDef &&
+  assert((!OldDef || OldDef == Def) &&
          "There can be only one definition of an opened archetype");
   OpenedArchetypeDefs[archetype] = Def;
 }