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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I mean, really, the issue here is the underlying instructions have functionality to set the trailing elements that isn't getting exposed by the intrinsic, and if the elements are always poison, there isn't any easy way to get access to that functionality.  If we don't think the functionality is important, that's fine... but if it is important, we should expose it as a "passthrough" input.  Trying to pattern-match it seems unreliable.

I'm not really familiar with the algorithms that use compress, so I'm not sure how important it is to set the trailing elements.

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