x/net/bpf: support jump conditions on `x`

[arthurfabre]

Currently conditionals / jumps are represented by JumpIf. This doesn't support conditionals / jumps that use register x instead of the constant k encoded in the instruction.

I have added preliminary support for this in the form of a new JumpIfX instruction.

Questions:

• Should JumpIf be renamed JumpIfConstant for consistency with ALUOpConstant & ALUOpX?
• Do I need to add support for this to the vm or is adding it to disassembler enough?

cc @danderson & @mdlayher as you seem to be the maintainers

[gopherbot]

Change https://golang.org/cl/136895 mentions this issue: bpf: Support JumpIf on RegX instead of K

[danderson]

Already reviewed your CL, but for reference...

I would prefer to not rename JumpIf, at least not until Go has generally available modules with good versioning. I don't want to break the API of every user of this package just for a little bit of consistency. Once Go modules are out of experimental state, we can look at maybe releasing a 2.0.0 of the module.

We want the VM to be able to execute any code that the asm/disasm knows how to handle. Your PR does include both, so that's a moot point now :).

Thank you for contributing the extra instruction! Mildly annoying that it's not documented (or did I just completely miss it out when I implemented the package originally??)