bpf: refactor inst patterns with more mnemonics

Currently, eBPF backend is using some constant directly in instruction patterns,
This patch replace them with mnemonics and removed some unnecessary temparary
variables.

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: 313958