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

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

1239537577ae406457928109d8d0cabe78ad588f enables the ADD verification assertions for PTRADD (with a comment to document why it is so overly restrictive) and a few straightforward early folds in `SelectionDAG::getNode()`.

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