SSA CFG: introduce projections for functions with multiple return values

[clonker]

• removes ValueId = (InstId, outputPos) in favor of plain InstIds
• multi-return-valued function calls (with >= 2 output values) now have
  • one Inst for the operation itself
  • as many projection Insts as they have outputs
• single-output ops keep their InstId as value handle

[blishko]

Looks good!

I have a few small comments.

[blishko]

Is this assuming that the projections have ids immediately following the id of the function call instruction?

[clonker]

Yeah exactly. I have elevated that to be an invariant of the SSA CFG. It's not like we can swap around projections and/or only extract a couple things anyways due to the stack nature of things. See here:

solidity/libyul/backends/evm/ssa/SSACFG.h

Lines 217 to 230 in 07053f4

	InstId makeCallWithProjections(
	BlockId const _block,
	Call _payload,
	std::vector<InstId> _inputs,
	InstructionStore::NumReturnsSizeType const _numReturns,
	langutil::DebugData::ConstPtr _debugData = {}
	)
	{
	InstId const producer = makeCall(_block, std::move(_payload), std::move(_inputs), _debugData);
	if (_numReturns >= 2)
	for (InstructionStore::NumReturnsSizeType i = 0; i < _numReturns; ++i)
	makeProjection(_block, producer, _debugData);
	return producer;
	}

[blishko]

Hopefully. I guess it would be good to check this invariant somewhere.
We should eventually have something like a pass to check that the IR is well-formed, right?

[clonker]

Yeah we should

[blishko]

LGTM!

I would like to have one more helper :)

[blishko]

Space missing? :)

[blishko]

Approved!