Auto merge of #105274 - saethlin:instcombine-mut-ref, r=cjgillot

Enable instcombine for mutable reborrows

`instcombine` used to contain this comment, which is no longer accurate because there it is fine to copy `&mut _` in MIR:
```rust
// The dereferenced place must have type `&_`, so that we don't copy `&mut _`.
```
So let's try replacing that check with something much more permissive...

Author: bors