[54015] NLL:Improve move error loop detection

[54015] NLL:Improve move error loop detection

Before this patch running the following command would generate the given output:
$ rustc +stage1 src/test/ui/liveness/liveness-move-in-while.rs -Zborrowck=mir -Ztwo-phase-borrows
error[E0382]: borrow of moved value: `y`
 --> src/main.rs:8:24
  |
8 |         println!("{}", y); //~ ERROR use of moved value: `y`
  |                        ^ value borrowed here after move
9 |         while true { while true { while true { x = y; x.clone(); } } }
  |                                                    - value moved here
  |
  = note: move occurs because `y` has type `std::boxed::Box<isize>`, which does not implement the `Copy` trait

We want to give the user more hint by telling them that the value was moved in the previous iteration of the
loop. After this patch, the error message adds the phrase "in previous iteration of loop" and in totality
looks like this:

$ rustc +stage1 src/test/ui/liveness/liveness-move-in-while.rs -Zborrowck=mir -Ztwo-phase-borrows
error[E0382]: borrow of moved value: `y`
  --> src/test/ui/liveness/liveness-move-in-while.rs:17:24
   |
17 |         println!("{}", y); //~ ERROR use of moved value: `y`
   |                        ^ value borrowed here after move
18 |         while true { while true { while true { x = y; x.clone(); } } }
   |                                                    - value moved here, in previous iteration of loop
   |
   = note: move occurs because `y` has type `std::boxed::Box<isize>`, which does not implement the `Copy` trait