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

[Craig Topper via llvm-commits]

topperc wrote:

Should we add these verification asserts to `getNode` like we have for `ISD::ADD`

```
    assert(VT.isInteger() && "This operator does not apply to FP types!");       
    assert(N1.getValueType() == N2.getValueType() &&                             
           N1.getValueType() == VT && "Binary operator types must match!");
```

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