[clang] [llvm] [AArch64] Add updated FEAT_SVE_B16B16 and begin replacement of 'b16b16' flag (PR #101480)

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https://github.com/CarolineConcatto approved this pull request.

Thank you Spencer for the work.
I left some comments about the sve2.1 feature flags being used for the sve2. But I dont think you need to change in this patch. Because it is unrelated
Maybe you create another patch a NFC to move all the sve2p1 tests and run lines to be only SVE2?
I believe when we changed the feature flag we forgot to move the tests and also to fix all run lines. 

https://github.com/llvm/llvm-project/pull/101480