[llvm] [CostModel][AArch64] Add initial costs for `masked.compressstore` (PR #213712)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> I'm assuming this is just a good first start to enable vectorisation and we'll revisit when we're in a better position to benchmark.

Yep, my motivation for this change is so I can add AArch64 tests for compress store vectorisation. 

https://github.com/llvm/llvm-project/pull/213712