dillon1000/react

Support OptionalCallExpression as LHS of LogicalExpression

A common idiom is to map over some possibly-missing list of items from a data 
payload and fall back to an empty array: 

```javascript 

const renderedItems = data?.items?.map(renderItem) ?? []; 

``` 

The way we were lowering OptionalCallExpression meant that in this case, we'd 
end up with an OptionalCallTerminal as the terminal of the logical expression's 
test block, which violates our internal invariant. Logical test blocks must end 
in a Branch! This PR fixes the immediate issue, which is that the callee - in 
this case `data?.items?.map` — was being lowered prior to the 
OptionalCallTerminal instead of inside its test block. Changing that fixes the 
shape of the IR and makes this example work. 

As part of investigating this I realized that the way I originally handled 
lowering of optional call isn't quite right. The difference isn't observable 
unless we did more sophisticated DCE but we don't correctly model the fact that 
if `data.items` is null that the `map()` call won't occur. That is technically 
fine bc we do model the fact that the `map()` call is conditional, and notably 
its arguments are only conditional dependencies. So it's good enough. But in a 
follow-up I'll change to model the fact that `data.items` is null, that the map 
call isn't reachable at all.