[llvm] [SelectionDAG] Introduce ISD::PTRADD (PR #140017)

[Fabian Ritter via llvm-commits]

================
@@ -1502,6 +1502,11 @@ enum NodeType {
   // Outputs: [rv], output chain, glue
   PATCHPOINT,
 
+  // PTRADD represents pointer arithmatic semantics, for targets that opt in
+  // using shouldPreservePtrArith().
+  // ptr = PTRADD ptr, offset
+  PTRADD,
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ritter-x2a wrote:

Currently, this PR just changes this to
```
def ptradd : SDNode<"ISD::PTRADD" , SDTPtrAddOp, []>;
```
When looking through the related code, I didn't have the impression that it mattered to the use in GlobalISel what the SDAG node here is, changing it to `ISD::PTRADD` at least doesn't break the test for the `V_ADD3_U32_e64` matching.

https://github.com/llvm/llvm-project/pull/140017