Study the Go compiler's poset data structure

[dominikh]

The Go compiler uses a partially ordered set and stack operations to track facts about SSA values at branch points. This presents an interesting alternative to e-SSA and SSI. We should definitely look into the pros and cons of this approach and how easy it would be to integrate with staticcheck.

https://github.com/golang/go/blob/7be97af2ff6d4bdc8a52cb72677bbd68703489fd/src/cmd/compile/internal/ssa/poset.go

Related: #339, #455, #362, #175