[llvm] [NFC][ARM] Split SIMD identifier away from MVE (PR #130296)

[Jack Styles via llvm-commits]

Stylie777 wrote:

@jthackray, yes as these are the only cores that enable MVE.FP. This paves the way for `+nosimd` being fully operational in a later patch, but currently MVE is reliant on the SIMD enum value, which is not ideal. So a new enum value is needed for these features, so SIMD can be handled by its own unique value. No other test cases will need changing here as there is no intended change in the behaviour.

I have updated the comment.

https://github.com/llvm/llvm-project/pull/130296