Function encode_insn_immediate() will be used by other instruction
manipulate related functions, so move it into insn.c and rename it
as aarch64_insn_encode_immediate().
Reviewed-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NJiang Liu <liuj97@gmail.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>