SILCombine: limit the worklist size for instruction canonicalization

To be precise: don't add instruction uses to the worklist if it already has more than 10000 elements.
This avoids quadratic behavior for very large functions.

rdar://problem/56268570

lib/SILOptimizer/SILCombiner/SILCombine.cpp

@@ -131,7 +131,9 @@ public:
   }
 
   void notifyHasNewUsers(SILValue value) override {
-    Worklist.addUsersToWorklist(value);
+    if (Worklist.size() < 10000) {
+      Worklist.addUsersToWorklist(value);
+    }
     changed = true;
   }