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

The i8 type is not registered with any register class.
This causes a segmentation fault in MachineLICM::getRegisterClassIDAndCost.

The code selects the first type associated with register class FPR8,
which happens to be i8.
It uses this type (i8) to get the representative class pointer, which is 0.
It then uses this pointer to access a field, resulting in segmentation fault.

Since i8 type is not being used for printing any neon instruction
we can safely remove it.