[MLIR] fix greedy pattern rewrite driver iteration on change

[bondhugula]

Summary

Removing dead ops should make the outer loop of the pattern rewriting
driver run again. Although its operands are added to the worklist, if no
changes happenned to them or remaining ops in the worklist, the driver
wouldn't run once again - but it should be.

mlir/lib/Transforms/Utils/GreedyPatternRewriteDriver.cpp

while (!worklist.empty()) {
       // them.
       if (op == nullptr)
         continue;
 
       // If the operation is trivially dead - remove it.
       if (isOpTriviallyDead(op)) {
         notifyOperationRemoved(op);
         op->erase();
+        changed = true;
         continue;
       }
 
       // Collects all the operands and result uses of the given `op` into work
       // list. Also remove `op` and nested ops from worklist.
       originalOperands.assign(op->operand_begin(), op->operand_end());
       auto preReplaceAction = [&](Operation *op) {
         // Add the operands to the worklist for visitation.
         addToWorklist(originalOperands);
 
         // Add all the users of the result to the worklist so we make sure
         // to revisit them.
         for (auto result : op->getResults())
           for (auto *operand : result.getUsers())
             addToWorklist(operand);
 
         notifyOperationRemoved(op);
       };
 
       // Try to fold this op.
       if (succeeded(folder.tryToFold(op, collectOps, preReplaceAction))) {
-        changed |= true;
+        changed = true;
         continue;
       }
 
       // Make sure that any new operations are inserted at this point.
       setInsertionPoint(op);
 
       // Try to match one of the patterns. The rewriter is automatically
       // notified of any necessary changes, so there is nothing else to do here.