nested loops with no block in between cause stack overflow in `Display`

[pufferfish101007]

When trying to compile the following code (not minimal)

a stack overflow occurs while trying to print the IR (so long as the 'print IR' setting is enabled). Otherwise we end up with similar stack-overflow-esque errors in either loop unrolling or SSA, depending on what is enabled.

This is not a regression from v0.0.8 as I originally thought.

[pufferfish101007]

This seems to be because generate_next_step does not respect the yield_first property of a NextBlockInfo, meaning that generate_next_step greedily inlines everything, causing a reference cycle.