[SILCombine] Keep dead drop_deinits.

Author: nate-chandler

lib/SILOptimizer/Utils/CanonicalizeInstruction.cpp

@@ -547,6 +547,8 @@ static SILBasicBlock::iterator
 eliminateUnneededForwardingUnarySingleValueInst(SingleValueInstruction *inst,
                                                 CanonicalizeInstruction &pass) {
   auto next = std::next(inst->getIterator());
+  if (isa<DropDeinitInst>(inst))
+    return next;
   for (auto *use : getNonDebugUses(inst)) {
     if (auto *destroy = dyn_cast<DestroyValueInst>(use->getUser())) {
       if (destroy->isFullDeinitialization())