Auto merge of #31286 - oli-obk:fix/mir_box, r=nagisa

the previous code generated a temporary of the inner type and assigned the box-memory to it. So if you did `let x: Box<usize> = box 5;` you got a

```rust
let var0: Box<usize>; // x
let mut tmp0: Box<usize>;
let mut tmp1: usize;
...
tmp1 = Box(usize);
(*tmp1) = const 5;
tmp0 = tmp1;
var0 = tmp0;
```

r? @nagisa

0 files changed, 0 insertions, 0 deletions