[llvm] [LLVM] Add `llvm.masked.compress` intrinsic (PR #92289)

[Lawrence Benson via llvm-commits]

lawben wrote:

> Maybe worth adding ISD::MCOMPRESS to VerifySDNode to ensure the types are correct?

@RKSimon that's a good idea. As `VerifySDNode` is still quite new, I'm not sure if this is intended behavior or not, but when implementing `MCOMPRESS` in it, I get crashes in my program. Specifically, in `SelectionDAG::getNode()`, there is a step that tries to `FoldConstantArithmetic`, which creates an `MCOMPRESS` node with scalar input. As this is not valid, my verify code crashes. I assume this will happen for many other nodes that require vector inputs.

I could add a check at the beginning of `FoldConstantArithmetic` and bail, like `CONCAT_VECTORS` 
https://github.com/llvm/llvm-project/blob/84aee95124549c5d13e22053af254e3fcc02bc84/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L6279-L6283

but that feels like the wrong place/abstraction to do this at. Do you have any suggestions where to put logic to avoid this? In my opinion, the crash in `VerifySDNode` is correct here and the scalar path should never be tried, so we probably want to avoid it.

https://github.com/llvm/llvm-project/pull/92289