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

[Lawrence Benson via llvm-commits]

lawben wrote:

> Why can't we check the types in getNode like we do for most nodes.

@topperc You are right. I've moved the code there now. I think it makes more sense there. I've also added code for constant folding, as some of the other instructions also do. 

@RKSimon I've also added a check in `FoldConstantArithmetic` to avoid `MCOMPRESS`, as this is now handled in `getNode()`.

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