[sil] Dump the unknown users when the address walker fails to visit an instructions.

Just making this easier to debug. Previously, one would have to go into the
debugger to figure out what the issue is. Now upon failure, one just gets the
exact instruction that needs to be added.

Author: gottesmm

lib/SIL/Verifier/SILVerifier.cpp

@@ -729,18 +729,30 @@ struct ImmutableAddressUseVerifier {
 };
 
 static void checkAddressWalkerCanVisitAllTransitiveUses(SILValue address) {
+  SmallVector<SILInstruction *, 8> badUsers;
   struct Visitor : TransitiveAddressWalker<Visitor> {
+    SmallVectorImpl<SILInstruction *> &badUsers;
+    Visitor(SmallVectorImpl<SILInstruction *> &badUsers)
+        : TransitiveAddressWalker<Visitor>(), badUsers(badUsers) {}
     bool visitUse(Operand *use) { return true; }
-    void onError(Operand *use) {}
+    void onError(Operand *use) {
+      badUsers.push_back(use->getUser());
+    }
   };
 
-  Visitor visitor;
+  Visitor visitor(badUsers);
   if (std::move(visitor).walk(address) != AddressUseKind::Unknown)
     return;
 
   llvm::errs() << "TransitiveAddressWalker walker failed to know how to visit "
                   "a user when visiting: "
-               << *address << '\n';
+               << *address;
+  if (badUsers.size()) {
+    llvm::errs() << "Bad Users:\n";
+    for (auto *user : badUsers) {
+      llvm::errs() << "    " << *user;
+    }
+  }
   llvm::report_fatal_error("invoking standard assertion failure");
 }