compiler: merge reactive scopes across const StoreLocal (#29095)

@jbonta nerd-sniped me into making this optimization during conference
prep, posting this as a PR now that keynote is over.

Consider these two cases:

```javascript
export default function MyApp1({ count }) {
  const cb = () => count;
  return <div onclick={cb}>Hello World</div>;
}

export default function MyApp2({ count }) {
  return <div onclick={() => count}>Hello World</div>;
}
```

Previously, the former would create two reactive scopes (one for `cb`,
one for the div) while the latter would only have a single scope for the
`div` and its inline callback. The reason we created separate scopes
before is that there's a `StoreLocal 'cb' = t0` instruction in-between,
and i had conservatively implemented the merging pass to not allow
intervening StoreLocal instructions.

The observation is that intervening StoreLocals are fine _if_ the
assigned variable's last usage is the next scope. We already have a
check that the intervening lvalues are last-used at/before the next
scope, so it's trivial to extend this to support StoreLocal.

Note that we already don't merge scopes if there are intervening
terminals, so we don't have to worry about things like conditional
StoreLocal, conditional access of the resulting value, etc.