They moved it to 1.7 because it didn't get completed in time. You can see the source code here: https://github.com/golang/go/tree/dev.ssa (In particular, https://github.com/golang/go/compare/dev.ssa is interesting).

It's not clear to me what work is left, but hopefully it gets merged super early in the 1.7 cycle.

That's interesting! PHP 7 is getting one too - https://marc.info/?l=php-internals&m=144990711202803&w=2 - i wonder which will be completed first?

That sounds like an (e-)SSA-based optimization but not like an SSA backend.