[llvm] [RFC][SelectionDAG] Add and use SDNode::getAsConstantVal() helper (PR #76710)

[Matt Arsenault via llvm-commits]

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@@ -923,6 +923,7 @@ END_TWO_BYTE_PACK()
 
   /// Helper method returns the integer value of a ConstantSDNode operand.
   inline uint64_t getConstantOperandVal(unsigned Num) const;
+  inline uint64_t getAsConstantVal() const;
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arsenm wrote:

Potentially confusing if you need a signed immediate, but that's already an issue with the existing helper 

https://github.com/llvm/llvm-project/pull/76710