dillon1000/react

[refactor] Refactor Memoize to two instructions: Start and Finish

--- 

Previously, we always emitted `Memoize dep` instructions after the function 
expression literal and depslist instructions 

```js 

// source 

useManualMemo(() => {...}, [arg]) 

// lowered 

$0 = FunctionExpression(...) 

$1 = LoadLocal (arg) 

$2 = ArrayExpression [$1] 

$3 = Memoize (arg) 

$4 = Call / LoadLocal 

$5 = Memoize $4 

``` 

Now, we insert `Memoize dep` before the corresponding function expression 
literal: 

```js 

// lowered 

$0 = StartMemoize (arg)      <---- this moved up! 

$1 = FunctionExpression(...) 

$2 = LoadLocal (arg) 

$3 = ArrayExpression [$2] 

$4 = Call / LoadLocal 

$5 = FinishMemoize $4 

``` 

Design considerations: 

- #2663 needs to understand which lowered instructions belong to a manual 
memoization block, so we need to emit `StartMemoize` instructions before the 
`useMemo/useCallback` function argument, which contains relevant memoized 
instructions 

- we choose to insert StartMemoize instructions to (1) avoid unsafe instruction 
reordering of source and (2) to ensure that Forget output does not change when 
enabling validation 

This PR only renames `Memoize` -> `Start/FinishMemoize` and hoists 
`StartMemoize` as described. The latter may help with stricter validation for 
`useCallback`s, although testing is left to the next PR. 

#2663 contains all validation changes