[llvm] [LLVM][CodeGen][SVE] Improve custom lowering for EXTRACT_SUBVECTOR. (PR #90963)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

The original commit was indeed broken for the cases where fixed length vectors are extracted from an unpacked scalable vector.  Fixing this uncovered other issues (hence the VECTOR_SPLICE PR) and I've upstream additional tests that would have caught the issue.  I've pushed the full chain to this PR for now but only the final commit is relevant.  Feel free to review but I'll keep the PR in draft until I can rebase to remove the dependent work.

https://github.com/llvm/llvm-project/pull/90963