dillon1000/react

RHS member expression converts to PropertyLoad

This is an incremental step to removing `Place.memberPath`. This PR changes how 
we handle MemberExpressions in rvalue position, converting to a new 
`PropertyLoad` InstructionValue variant. Example: 

``` 

let x = a.b; 

x.y = b.c; 

=> 

Const tmp1 = PropertyLoad a, 'b'; 

Const x = Place tmp1; 

Const tmp2 = PropertyLoad b, 'c'; 

Reassign x.y = tmp2 

``` 

That we already recently made a chance to ensure that _if_ the lvalue is a 
member expression, that we convert the RHS to a Place. So although `x.y = b.c` 
could technically be lowered to a single instruction (with the`b.c` as a 
PropertyLoad), we force this to a temporary to ensure that we can independently 
memoize the RHS value. 

The net result is that the following combinations are possible: 

* `x = y`, lvalue identifier, rvalue identifier 

* `x.y = y` lvalue member path, rvalue identifier 

* `x = y.z` lvalue identifier, rvalue property load 

As noted above, `x.y = a.b` no longer occurs (and there's an invariant for this 
in one of the passes). 

A follow-up PR will add a PropertyStore instruction so that we can remove member 
paths in lvalue position too.