Add a JumpIfX instruction which implements conditional jumps using
RegA and RegX. This is in addition to the pre-existing JumpIf
instruction which uses RegA and K.

This instruction / addressing mode is not mentionned in the original BPF
paper, but is supported by tools like bpf_asm, and has recently been
added to the kernel's filter.txt.

Simplify some of the parsing logic, and add a separate helper for
checking for "fake" JumpIfs.

Add JumpIfX support to the BPF vm.

Update testdata with JumpIfX instructions, and add tests
for both the assembler/disassembler and vm.

Fixes golang/go#27814

Change-Id: I0c3f6ac7eb5b4cd4d9c5af8784ee2e8d25195a0a
GitHub-Last-Rev: 39a88165b2d3253c37db4b0e303d862b60dc37c9
GitHub-Pull-Request: golang/net#20
Reviewed-on: https://go-review.googlesource.com/c/136895Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>