[AArch64] Removed unused i8 type from FPR8 register class.

[Tim Northover]

Hi Ana,

> -        the current patch removes i8 from FPR8 since it is not used anyways
> (at least at the moment).

Removing it sounds like a good idea if it's not being used (which I'd
hope it isn't since i8 isn't legal).

Cheers.

Tim.