[SelectionDAG] Deprecate isNullValue and isAllOnesValue

[kazu]

Summary

This patch deprecates them as there are no known uses of these
functions in the project.

[dblaikie]

Sure, sounds good. (maybe document the history of how we ended up with duplicate functionality/different names for the same thing? (I haven't checked the history - if you introduced the new names, maybe link to where they were introduced and/or summarize why they were introduced, etc))

[MaskRay]

I think you can just remove the methods. They seem pretty uncommon and downstream projects can simply migrate uses when integrating this patch.

[MaskRay]

The isAllOnesValue name may come from Constant.

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

public:
   uint64_t getLimitedValue(uint64_t Limit = UINT64_MAX) {
     return Value->getLimitedValue(Limit);
   }
   MaybeAlign getMaybeAlignValue() const { return Value->getMaybeAlignValue(); }
   Align getAlignValue() const { return Value->getAlignValue(); }
 
   bool isOne() const { return Value->isOne(); }
   bool isZero() const { return Value->isZero(); }
-  // NOTE: This is soft-deprecated.  Please use `isZero()` instead.
+  LLVM_DEPRECATED("use isZero instead", "isZero")
   bool isNullValue() const { return isZero(); }
   bool isAllOnes() const { return Value->isMinusOne(); }
-  // NOTE: This is soft-deprecated.  Please use `isAllOnes()` instead.
+  LLVM_DEPRECATED("use isAllOnes instead", "isAllOnes")
   bool isAllOnesValue() const { return isAllOnes(); }
   bool isMaxSignedValue() const { return Value->isMaxValue(true); }
   bool isMinSignedValue() const { return Value->isMinValue(true); }
 
   bool isOpaque() const { return ConstantSDNodeBits.IsOpaque; }
 
   static bool classof(const SDNode *N) {
     return N->getOpcode() == ISD::Constant ||