Remove Engine::new_gen_kill.

This is an alternative to `Engine::new_generic` for gen/kill analyses.
It's supposed to be an optimization, but it has negligible effect.
The commit merges `Engine::new_generic` into `Engine::new`.

This allows the removal of various other things: `GenKillSet`,
`gen_kill_statement_effects_in_block`, `is_cfg_cyclic`.

compiler/rustc_mir_dataflow/src/framework/mod.rs

@@ -242,7 +242,7 @@ pub trait Analysis<'tcx>: AnalysisDomain<'tcx> {
     where
         Self: Sized,
     {
-        Engine::new_generic(tcx, body, self)
+        Engine::new(tcx, body, self)
     }
 }
 
@@ -376,19 +376,6 @@ where
     ) {
         self.switch_int_edge_effects(block, discr, edge_effects);
     }
-
-    /* Extension methods */
-    #[inline]
-    fn into_engine<'mir>(
-        self,
-        tcx: TyCtxt<'tcx>,
-        body: &'mir mir::Body<'tcx>,
-    ) -> Engine<'mir, 'tcx, Self>
-    where
-        Self: Sized,
-    {
-        Engine::new_gen_kill(tcx, body, self)
-    }
 }
 
 /// The legal operations for a transfer function in a gen/kill problem.
@@ -422,44 +409,6 @@ pub trait GenKill<T> {
     }
 }
 
-/// Stores a transfer function for a gen/kill problem.
-///
-/// Calling `gen_`/`kill` on a `GenKillSet` will "build up" a transfer function so that it can be
-/// applied multiple times efficiently. When there are multiple calls to `gen_` and/or `kill` for
-/// the same element, the most recent one takes precedence.
-#[derive(Clone)]
-pub struct GenKillSet<T> {
-    gen_: HybridBitSet<T>,
-    kill: HybridBitSet<T>,
-}
-
-impl<T: Idx> GenKillSet<T> {
-    /// Creates a new transfer function that will leave the dataflow state unchanged.
-    pub fn identity(universe: usize) -> Self {
-        GenKillSet {
-            gen_: HybridBitSet::new_empty(universe),
-            kill: HybridBitSet::new_empty(universe),
-        }
-    }
-
-    pub fn apply(&self, state: &mut impl BitSetExt<T>) {
-        state.union(&self.gen_);
-        state.subtract(&self.kill);
-    }
-}
-
-impl<T: Idx> GenKill<T> for GenKillSet<T> {
-    fn gen_(&mut self, elem: T) {
-        self.gen_.insert(elem);
-        self.kill.remove(elem);
-    }
-
-    fn kill(&mut self, elem: T) {
-        self.kill.insert(elem);
-        self.gen_.remove(elem);
-    }
-}
-
 impl<T: Idx> GenKill<T> for BitSet<T> {
     fn gen_(&mut self, elem: T) {
         self.insert(elem);