mem: Avoid setting markPending if not needed

In cases where a newly added target does not have any upstream MSHR to
mark as downstreamPending, remember that nothing is marked. This
allows us to avoid attempting to find the MSHR as part of the clearing
of downstreamPending.

src/mem/cache/mshr.cc

@@ -101,6 +101,9 @@ MSHR::TargetList::add(PacketPtr pkt, Tick readyTime,
         if (mshr != NULL) {
             assert(!mshr->downstreamPending);
             mshr->downstreamPending = true;
+        } else {
+            // No need to clear downstreamPending later
+            markPending = false;
         }
     }