bpf: refactor inst patterns with better inheritance

Arithmetic and jump instructions, load and store instructions are sharing
the same 8-bit code field encoding,

A better instruction pattern implemention could be the following inheritance
relationships, and each layer only encoding those fields which start to
diverse from that layer. This avoids some redundant code.

  InstBPF -> TYPE_ALU_JMP -> ALU/JMP
  InstBPF -> TYPE_LD_ST -> Load/Store

Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Yonghong Song <yhs@fb.com>
llvm-svn: 313959