Revert "[sil-optimizer] Remove the opened archetypes related workarounds which are obsolete now."

This reverts commit 78683186f1.

It is related to 8ef8bb4eb1 which broke
swift_tools-RA_stdlib-RD_test-no_device and some others.

lib/SILOptimizer/Transforms/Sink.cpp

@@ -67,6 +67,19 @@ public:
     if (II->isAllocatingStack() || II->isDeallocatingStack())
       return false;
 
+    // We don't sink open_existential_* instructions, because
+    // there may be some instructions depending on them, e.g.
+    // metatype_inst, etc. But this kind of dependency
+    // cannot be expressed in SIL yet.
+    switch (II->getKind()) {
+    default: break;
+    case ValueKind::OpenExistentialBoxInst:
+    case ValueKind::OpenExistentialRefInst:
+    case ValueKind::OpenExistentialAddrInst:
+    case ValueKind::OpenExistentialMetatypeInst:
+      return false;
+    }
+
     SILBasicBlock *CurrentBlock = II->getParent();
     SILBasicBlock *Dest = nullptr;
     unsigned InitialLoopDepth = LoopInfo->getLoopDepth(CurrentBlock);