stack_ supports all primitives that returns poly return mode

[riaqn]

This PR allows writing stack_ (ref "hello").

In general, ref could be any primitive whose return mode is poly. We already implicitly optimize allocation for these case, so stack_ should inherit that behavior.

[goldfirere]

I see why we might want this, but was this change motivated by a concrete use? In addition, a return mode of poly isn’t always right — for example, %identity has that property, I believe, but isn’t an allocation. If we wanted to do this, I think it would be better just to list out the primitives that optionally stack-allocate. But I probably favor not doing this at all, but instead to make the stack-allocation apparent in the named external, like ref_stack. If someone wants to ensure a stack allocation, they just use ref_stack.

[goldfirere]

OK — I found the Slack thread that provides more context (and that already suggests not to use the “poly” criterion). But I still think that we should just have a stack_ref instead of allowing this. Let’s continue the design debate on Slack.