[llvm] [LoongArch] Fix MergeBaseOffset for constant pool index operand (PR #159336)

[via llvm-commits]

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@@ -788,6 +788,9 @@ class MachineOperand {
   LLVM_ABI void ChangeToBA(const BlockAddress *BA, int64_t Offset,
                            unsigned TargetFlags = 0);
 
+  /// ChangeToCPI - Replace this operand with a new constant pool index operand.
+  LLVM_ABI void ChangeToCPI(unsigned Idx, int Offset, unsigned TargetFlags = 0);
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zhaoqi5 wrote:

Is it better to use `int Offset` or `int64_t Offset` like other hooks?

https://github.com/llvm/llvm-project/pull/159336