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

[Tomas Matheson via llvm-commits]

tmatheson-arm wrote:

So, while "FEAT_SVE2 is OPTIONAL from Armv9.0", the PCS requires that v9.0-a has SVE2. From the Arm ARM:

> All Armv9-A systems that support standard operating systems with rich application environments also provide hardware support for SVE2 instructions. It is a requirement of the ARM Procedure Call Standard for AArch64, see Procedure Call Standard for the Arm 64-bit Architecture.

I think this is ok to do, as it keeps SVE2 enabled by default for `-march=armv9-a`. However for any downstream users of LLVM, `"target-features"="+v9a"` will not get them SVE2, they will need to explicitly add `"+v9a,+sve2"`. That seems ok to me, but I'm not 100% sure.

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