i#6938 migrate: Fix observed_migrations assert in schedule_stats

Fixes an assert on the new observed_migrations stat added to
schedule_stats in PR #7057.  These observed_migrations are counted on
the destination core, while the scheduler reports migrations away from
a source core: so they can differ, causing the assert to fire.  Fixed
by moving it to only check the aggregated stats across all cores.

Tested on the internal trace where the assert fired before.

Issue: #6938

clients/drcachesim/tools/schedule_stats.cpp

@@ -498,15 +498,13 @@ schedule_stats_t::aggregate_results(counters_t &total)
         assert(shard.second->counters.direct_switches ==
                shard.second->stream->get_schedule_statistic(
                    memtrace_stream_t::SCHED_STAT_DIRECT_SWITCH_SUCCESSES));
-        // If the scheduler is counting migrations, it may see more due to inputs
-        // not yet executed moving among runqueues.
-#ifndef NDEBUG
-        double sched_migrations = shard.second->stream->get_schedule_statistic(
-            memtrace_stream_t::SCHED_STAT_MIGRATIONS);
-#endif
-        assert(sched_migrations == 0. ||
-               sched_migrations >= shard.second->counters.observed_migrations);
     }
+    // Our observed_migrations are counted on the destination core, while
+    // the scheduler reports migrations away from a source core: so we only
+    // check the aggregate. For non-dynamic schedules the scheduler-reported
+    // will be 0; otherwise, the scheduler may see more migrations due to inputs
+    // not yet executed moving among runqueues.
+    assert(total.migrations == 0. || total.migrations >= total.observed_migrations);
 }
 
 bool