Tweak GenKillAnalysis method arguments.

`GenKillAnalysis` has very similar methods to `Analysis`, but the first
two have a notable difference: the second argument is `&mut impl
GenKill<Self::Idx>` instead of `&mut Self::Domain`. But thanks to the
previous commit, this difference is no longer necessary.

compiler/rustc_mir_dataflow/src/impls/liveness.rs

@@ -50,7 +50,7 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeLiveLocals {
 
     fn statement_effect(
         &mut self,
-        trans: &mut impl GenKill<Self::Idx>,
+        trans: &mut Self::Domain,
         statement: &mir::Statement<'tcx>,
         location: Location,
     ) {