instructions: support REP prefix

[mmcloughlin]

@klauspost has expressed interest in using the REP prefix string instructions.

https://github.com/klauspost/compress/pull/186/files#diff-60d6e6209d2fc7873851f07b946a5dc7R685

Related #85

[mmcloughlin]

#104 is a useful POC, thanks for the contribution @klauspost. I need to do some more research about how the Go assembler and opcode database handle these instructions, and it might also be useful to see how PeachPy represents them.

Some initial thoughts on the PR #104 itself:

• I would prefer to keep the original Maratyszcza/Opcodes data file untouched, as it should make it easier to incorporate future updates to that repo. We could still follow the approach in Very basic REP+MOVS support #104 by having an extras.xml or something with the same schema.
• These forms don't require register arguments in Go assembly (for example REP; MOVSB in runtime/memmove_amd64.s) so they shouldn't in avo either. We would need to use the implicit operands feature here.

Anyway, I need to do some more digging before I can decide on the cleanest way to support this.

[mmcloughlin]

From the Maratyszcza/Opcodes readme:

Specifically: "Legacy string/streaming instructions (MOVS/SCAS/CMPS/STOS/LODS)"
and "LOCK and REP/REPZ/REPNZ prefixes" are not supported.

avo uses this database as the primary source of instructions, therefore they are currently missing.

[dgryski]

Another argument in favor of switching to Intel's data files?

[mmcloughlin]

Another argument in favor of switching to Intel's data files?

I'm definitely going to have to bite the bullet on this at some point! #23

The asmjit/asmdb database backing asmjit is also quite nice, providing the necessary information for this feature and the LOCK prefix #85. This could be a short term solution preferable to just adding instructions by hand.

https://github.com/asmjit/asmdb/blob/9de003dbe8e28971dfdb9d6345a2b9f0b61f7c92/x86data.js#L185-L202

[klauspost]

These forms don't require register arguments in Go assembly.

I used BLENDVPS as a template, which has xmm0 as an implicit parameter, but it is still sent as parameter. Adding it like this will at least make it clear to the assembler there is a dependency on it and IMO makes it a tiny bit easier to use.

[mmcloughlin]

@klauspost Is this issue also blocking you?

[klauspost]

@mmcloughlin No. I can work around this. Thanks for asking!

[cocotyty]

After four years , avo still dose not support rep instruction.

Here is a demo for some guys who are looking for how to use rep instruction with avo:

	Load(Param("dest").Base(), reg.RDI)
	Load(Param("src").Base(), reg.RSI)
	Load(Param("length"), reg.RCX)
	Instruction(&ir.Instruction{Opcode: "REP"})
	Instruction(&ir.Instruction{Opcode: "MOVSB"})