bpf: disassembler support for XADD under sub-register mode

Like the other load/store instructions, "w" register is preferred when
disassembling BPF_STX | BPF_W | BPF_XADD.

v1 -> v2:
 - Updated testcase insn-unit.s (Yonghong)

Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
llvm-svn: 355127