[AArch64] Legalize MVT::i64x8 in DAG isel lowering

This patch legalizes the Machine Value Type introduced in D94096 for loads
and stores. A new target hook named getAsmOperandValueType() is added which
maps i512 to MVT::i64x8. GlobalISel falls back to DAG for legalization.

Differential Revision: https://reviews.llvm.org/D94097