bpf: New decoder namespace for 32-bit subregister load/store

When -mattr=+alu32 passed to the disassembler, use decoder namespace for
32-bit subregister.

This is to disassemble load and store instructions in preferred B format
as described in previous commit:

      w = *(u8 *) (r + off) // BPF_LDX | BPF_B
      w = *(u16 *)(r + off) // BPF_LDX | BPF_H
      w = *(u32 *)(r + off) // BPF_LDX | BPF_W

      *(u8 *) (r + off) = w // BPF_STX | BPF_B
      *(u16 *)(r + off) = w // BPF_STX | BPF_H
      *(u32 *)(r + off) = w // BPF_STX | BPF_W

NOTE: all other instructions should still use the default decoder
      namespace.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Yonghong Song <yhs@fb.com>
llvm-svn: 325990