[flang] Do not fold fir.box_addr when it has a slice

This patch avoids to fold `fir.box_addr` when the defining `fir.embox` op
has a slice. If the op is folded the slice information are lost.

This kind of problem occurred with code like:

```
call check(y(half+1:))
```

where `y` is an array.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D123392