[llvm] [llvm][AArch64] SVE2 is an optional feature in ARMv9.0a (PR #96007)

[Tomas Matheson via llvm-commits]

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

LGTM. Might need a release note, to document the backend `target-feature` dependency change? But I had a quick look and couldn't see any examples of similar changes in the release notes.

https://github.com/llvm/llvm-project/pull/96007