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

This reverts commit 7868318.

It is related to 8ef8bb4 which broke
swift_tools-RA_stdlib-RD_test-no_device and some others.

Author: Xin Tong

Diff for: lib/SILOptimizer/LoopTransforms/LICM.cpp

@@ -231,6 +231,13 @@ static bool canHoistInstruction(SILInstruction *Inst, SILLoop *Loop,
   if (isa<AllocationInst>(Inst) || isa<DeallocStackInst>(Inst))
     return false;
 
+  // Can't hoist metatype instruction referring to an opened existential,
+  // because it may break the dominance relationship.
+  if (isa<MetatypeInst>(Inst) &&
+      Inst->getType().getSwiftRValueType()->hasOpenedExistential()) {
+    return false;
+  }
+
   // Can't hoist instructions which may have side effects.
   if (!hasNoSideEffect(Inst, SafeReads))
     return false;

Diff for: lib/SILOptimizer/Transforms/Sink.cpp

@@ -67,6 +67,19 @@ class CodeSinkingPass : public SILFunctionTransform {
     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);