[llvm] [AArch64] Avoid introducing illegal types in LowerVECTOR_COMPRESS (NFC) (PR #168520)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

I'm not going to pick at this PR because from what I can see `LowerVECTOR_COMPRESS` is already poorly implemented and you're not making it much worse.  FYI: I've made a mental note to revisit this function to see if I can clean it up.

https://github.com/llvm/llvm-project/pull/168520