[rust-compiler] Preserve ref access location across phi joins (#37524)

Preserve the RefValue source location when joining mixed ref types so
validation errors point to the original ref access.

Before this change it used to show the incorrect location when using the
rust compiler.

```rust
  2 |   const ref = useRef(null);
  3 |   const x = cond ? ref : ref.current;
> 4 |   return <Foo value={x} />;
    |                      ^ Cannot access ref value during render
  5 | }
  6 |
```