dillon1000/react

[hir] Refactor useMemo inlining

Previously useMemo inlining created a new StoreLocal assignment (not 
reassignment!) instruction for every return value. This breaks when the return 
is inside a block (like an if-block) as the  scope is tied to the block. 

For example: ``` let x = useMemo(() => {   if (...) {     return { ... };   } }) 
``` would become: ``` if (...) {   const temp = { ... }; } const x = temp; ``` 

This PR instead changes the inlining to declare a temporary in the function 
prologue and then reassign values to it when replacing return statements. 

``` let x = useMemo(() => {   if (...) {     return { ... };   } }) ``` 

becomes 

``` let temp; if (...) {   temp = { ... }; } const x = temp; ```