Enable comparison between OperationName and StringLiteral (NFC)

[mehdi_amini]

Summary

This allows to write:

if (op->getName() == "some.op")

instead of

if (op->getName().getStringRef() == "some.op")

[jpienaar]

Isn't this a bit of a feature that doing the string comparison needs to be explicit? It is less efficient than the compares currently exposed. Well modulo getting an Identifier requires adding it to context, but the user has control and the choice is explicit. Would a wrapper class work here to both make the writing simpler and (potentially) cache for efficient further checking post first hit?

mlir/include/mlir/IR/OperationSupport.h

 inline bool operator==(OperationName lhs, OperationName rhs) {
   return lhs.getAsOpaquePointer() == rhs.getAsOpaquePointer();
 }
 
 inline bool operator!=(OperationName lhs, OperationName rhs) {
   return lhs.getAsOpaquePointer() != rhs.getAsOpaquePointer();
 }
 
+inline bool operator==(OperationName lhs, StringRef rhs) {
+  return lhs.getStringRef() == rhs;
+}
+
+inline bool operator!=(OperationName lhs, StringRef rhs) {
+  return lhs.getStringRef() != rhs;
+}
+
 // Make operation names hashable.
 inline llvm::hash_code hash_value(OperationName arg) {
   return llvm::hash_value(arg.getAsOpaquePointer());
 }
 
 //===----------------------------------------------------------------------===//
 // OperationState
 //===----------------------------------------------------------------------===//