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

Author: eeckstein

include/swift/SIL/SILInstructionWorklist.h

@@ -72,6 +72,9 @@ class SILInstructionWorklist : SILInstructionWorklistBase {
   /// Returns true if the worklist is empty.
   bool isEmpty() const { return worklist.empty(); }
 
+  /// Returns the number of elements in the worklist.
+  unsigned size() const { return worklist.size(); }
+
   /// Add the specified instruction to the worklist if it isn't already in it.
   void add(SILInstruction *instruction) {
     if (worklist.insert(instruction).second) {

lib/SILOptimizer/SILCombiner/SILCombine.cpp

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