[llvm] [AArch64] Lower vector.[de]interleave4 (PR #141513)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

Rather than each target having to do this can you move the expansion into the target neutral part of operation legalisation? with `AArch64TargetLowering::LowerVECTOR_DEINTERLEAVE` returning `SDValue()` for the cases where expansion is required.

This will make it easier to support the other interleave factors as well because the common expansion can just expand one level down (i.e. (de)interleave8 -> (de)interleave4, then (de)interleave4 -> (de)interleave2), which gives a target the option to say the intermediate step is legal or to custom lower it.

https://github.com/llvm/llvm-project/pull/141513