[ownership] Change ForwardingOperand::visitForwardingValues to handle ForwardingOwned terminators that are function exiting correctly.

Previously, we would have just asserted when we passed the operand to
PhiOperand on the next line of the code.

Author: gottesmm

lib/SIL/Utils/OwnershipUtils.cpp

@@ -1612,6 +1612,11 @@ bool ForwardingOperand::visitForwardedValues(
         });
   }
 
+  // If our terminator is function exiting, we do not have a value to visit, so
+  // just return.
+  if (ti->isFunctionExiting())
+    return true;
+
   auto *succArg = PhiOperand(use).getValue();
   return visitor(succArg);
 }