[AArch64] Add a GPR64x8 register class

This adds a GPR64x8 register class that will be needed as the data
operand to the LD64B/ST64B family of instructions in the v8.7-A
Accelerator Extension, which load or store a contiguous range of eight
x-regs. It has to be its own register class so that register allocation
will have visibility of the full set of registers actually read/written
by the instructions, which will be needed when we add intrinsics and/or
inline asm access to this piece of architecture.

Patch written by Simon Tatham.

Reviewed By: ostannard

Differential Revision: https://reviews.llvm.org/D91774