[RFC][AArch64] Define TARGET_SPILL_CLASS

[Ramana Radhakrishnan]

>> I don't think increasing GP2FP and FP2GP costs is a bad thing. In a
>> number of benchmarks we've seen increased moves between FP and integer
>> registers and having this fix appears to help some of them. However
>> moving this to generic model needs more benchmarking across a variety
>> of cores before it can safely be applied there.
>
> I'm aligned with Richards earlier comment on this topic.  Specifically
> I'd like to see numbers in processor specific tuning tables, then we
> can take a view on how to adjust the generic numbers.

Agreed - that's what I wanted to say but probably wasn't clear enough.

Ramana

>
> /Marcus