Extend max register size to accommodate AArch64 SVE vector regs

Summary: This patch increases maximum register size to 256 bytes to accommodate AArch64 SVE registers maximum possible size of 256 bytes.

Reviewers: labath, jankratochvil, rengolin

Reviewed By: labath

Subscribers: tschuett, kristof.beyls, danielkiss, lldb-commits

Differential Revision: https://reviews.llvm.org/D77044