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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

The only thing I'm still wondering about is whether there should be an initial 'background' value argument as well - currently all remaining upper elements are assumed to be undef, but I often at least need these to be zero. Creating a post-selection mask that is compatible with the compression mask can be rather annoying.

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