[llvm] [SelectionDAG] Preserve volatile undef stores. (PR #99918)

[Matt Davis via llvm-commits]

enferex wrote:

> Should this be backpeoted to 19.x? Since something as big as volatile could have huge effects on codegen?

@RSilicon , this "issue" of the DAGCombiner dropping volatile undef stores has been around for quite a while, 10+years.  I'm not sure we want to back port this, I'm happy to do so, I do see one [bug ](https://github.com/llvm/llvm-project/issues/27683).  However, given it's been around for a while,  I can also see where it makes sense to just take it on as a fix for 20.  

https://github.com/llvm/llvm-project/pull/99918