[SelectionDAG] Preserve volatile undef stores. (#99918)
This patch preserves `undef` SDNodes that are `volatile` qualified.
Previously, these nodes would be discarded. The motivation behind this
change is to adhere to the
[LangRef](https://llvm.org/docs/LangRef.html#volatile-memory-accesses),
even though that doc is mostly in terms of LLVM-IR, it seems reasonable
to imply that the volatile constraints also imply to SDNodes.
> Certain memory accesses, such as
[load](https://llvm.org/docs/LangRef.html#i-load)’s,
[store](https://llvm.org/docs/LangRef.html#i-store)’s, and
[llvm.memcpy](https://llvm.org/docs/LangRef.html#int-memcpy)’s may be
marked volatile. The optimizers must not change the number of volatile
operations or change their order of execution relative to other volatile
operations. The optimizers may change the order of volatile operations
relative to non-volatile operations. This is not Java’s “volatile” and
has no cross-thread synchronization behavior.
Source: https://llvm.org/docs/LangRef.html#volatile-memory-accesses
