[MoveOnlyAddressChecker] Handle open_exi_addr.

It's a pass-through for FSPL's purposes.

Author: nate-chandler

lib/SIL/Utils/FieldSensitivePrunedLiveness.cpp

@@ -158,6 +158,12 @@ SubElementOffset::computeForAddress(SILValue projectionDerivedFromRoot,
       continue;
     }
 
+    if (auto *oea =
+            dyn_cast<OpenExistentialAddrInst>(projectionDerivedFromRoot)) {
+      projectionDerivedFromRoot = oea->getOperand();
+      continue;
+    }
+
     if (auto *teai =
             dyn_cast<TupleElementAddrInst>(projectionDerivedFromRoot)) {
       SILType tupleType = teai->getOperand()->getType();