Range notation not handled as expected for `binop_separator="Back"` configuration

[davidalber]

This was identified in #2361.

This issue affects formatting for range notation with binop_separator="Back".

Reproduction:

1. Create a rustfmt.toml containing binop_separator="Back".
2. Run rustfmt on the following file.

fn main() {
    let range = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa..
        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb;
}

Expected:
No diff after formatting.

Actual:
The operator is moved to the front.

 fn main() {⏎
-    let range = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa..⏎
-        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb;⏎
+    let range = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa⏎
+        ..bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb;⏎
 }⏎
 ⏎

[davidalber]

Fixed by #2367.