dillon1000/react

BuildHIR for OptionalCallExpression

Implements lowering for a subset of optional calls - specifically, we don't 
(yet) support when the callee is a member expression or an optional member 
expression. So `foo?.()` works but we bailout on `object?.foo()` and 
`object.foo?.()`. 

For `<calleee>?.(<args>)` we lower as roughly: 

``` 

bb0: 

t0 = <callee> 

OptionalCall test=bb1 fallthrough= 

bb1 (value): 

Branch t0 consequent=bb2 alternate=bb3 

bb2 (value): 

...lower <args> here... 

t1 = Call t0, args 

StoreLocal res, t1 

Goto bb4 

bb3 (value): 

t2 = undefined 

StoreLocal res, t2 

Goto bb4 

bb4: 

// result in `res` here 

```