Add $_op hook to RewriterGen.

[merge_guards_bot]

Unit tests: unknown.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt

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mlir/tools/mlir-tblgen/RewriterGen.cpp

 void PatternEmitter::emitMatchLogic(DagNode tree) {
   LLVM_DEBUG(llvm::dbgs() << "--- start emitting match logic ---\n");
   emitOpMatch(tree, 0);
 
   for (auto &appliedConstraint : pattern.getConstraints()) {
     auto &constraint = appliedConstraint.constraint;
     auto &entities = appliedConstraint.entities;
 
+    fmtCtx.withOp("op");
+
     auto condition = constraint.getConditionTemplate();
     auto cmd = "if (!({0})) return matchFailure();\n";
 
     if (isa<TypeConstraint>(constraint)) {
       auto self = formatv("({0}.getType())",
                           symbolInfoMap.getValueAndRangeUse(entities.front()));
       os.indent(4) << formatv(cmd,
                               tgfmt(condition, &fmtCtx.withSelf(self.str())));