Consider growing stacks in compiler instead of iterative lowering

[yupferris]

I was recently made aware of this, unfortunately after converting lowering to iterative rather than recursive code!

Ofc both approaches have tradeoffs, but the code is probably overall simpler if we conditionally grow stacks, which is very important - so we may want to consider going back to the older pattern instead.

[yupferris]

I actually did a quick test of this for sim::ir::Expr::write, and ended up with an ICE, so I backed out - this should be properly isolated/reported when I have more time/motivation to look into it!