[MLIR] Define an order on `Value`s

[frgossen]

Summary

A default order on Values allows for its use in, e.g.,
llvm::EquivalenceClasses.

[jpienaar]

This order won't be stable between runs though, so any test depends on the ordering would be flaky. Why not add a simple wrapper class locally where you need this? Keeps it local to where the ordering doesn't matter and avoids folks accidentally relying on it accidentally.

[herhut]

I typically use getAsOpaquePointer() to get a void* and then place that in the collection at hands. That is unstable too but only locally where the context is clear.

[frgossen]

Thanks! Will do that

mlir/include/mlir/IR/Value.h

public:
   void *getAsOpaquePointer() const { return ownerAndKind.getOpaqueValue(); }
   static Value getFromOpaquePointer(const void *pointer) {
     Value value;
     value.ownerAndKind.setFromOpaqueValue(const_cast<void *>(pointer));
     return value;
   }
 
   friend ::llvm::hash_code hash_value(Value arg);
+  bool operator<(const Value &rhs) const {
+    return ownerAndKind.getOpaqueValue() < rhs.ownerAndKind.getOpaqueValue();
+  }
 
 protected:
   /// Returns true if the given operation result can be packed inline.
   static bool canPackResultInline(unsigned resultNo) {
     return resultNo < static_cast<unsigned>(Kind::TrailingOpResult);
   }
 
   /// Construct a value.