[llvm] [AArch64] Add lowering for `@llvm.experimental.vector.compress` (PR #101015)

[Lawrence Benson via llvm-commits]

lawben wrote:

Converting back to a regular PR, as this now does the correct thing. 

I tried to implement this in GlobalISel too, but there seem to be some issues with SVE there and non-legal types. So I cannot implement the same behavior in both SelectionDAG and GlobalISel. Does it make sense to skip GlobalISel for now or implement a part of the feature and then add stuff later? This can also be in a separate PR.

https://github.com/llvm/llvm-project/pull/101015