Remove a FIXME comment.

It is possible to avoid the clone as suggested in the comment. It would
require introducing an enum with two variants
`CloneBeforeModifying(&Domain)` and `Modifiable(&mut Domain)`. But it's
not worth the effort, because this code path just isn't very hot. E.g.
when compiling a large benchmark like `cargo-0.60.0` it's only hit a few
thousand times.

compiler/rustc_mir_dataflow/src/framework/direction.rs

@@ -76,8 +76,6 @@ impl Direction for Backward {
         for pred in body.basic_blocks.predecessors()[block].iter().copied() {
             match body[pred].terminator().kind {
                 // Apply terminator-specific edge effects.
-                //
-                // FIXME(ecstaticmorse): Avoid cloning the exit state unconditionally.
                 mir::TerminatorKind::Call { destination, target: Some(dest), .. }
                     if dest == block =>
                 {