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

[Lawrence Benson via llvm-commits]

lawben wrote:

@RKSimon I think this now covers the `passthru` options listed above. The "undef" path is unchanged, the "all zeros" path is covered by checking for a splat vector, and the "other" case is the `HasPassthru` path in `TargetLowering::expandMASKED_COMPRESS()`. Implemented and tested in both SelectionDAG and GlobalISel. GlobalIsel will receive a bit more attention in the follow-up PR for AArch64, where I explicitly handle il-/legal cases.

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