Document assumptions made in generator transform for analyses

The generator transform needs to inspect all possible dataflow states.
This can be done with half the number of bitset union operations if we
can assume that the relevant analyses do not use "before" effects.

src/librustc_mir/dataflow/impls/borrowed_locals.rs

@@ -99,6 +99,9 @@ impl<K> GenKillAnalysis<'tcx> for MaybeBorrowedLocals<K>
 where
     K: BorrowAnalysisKind<'tcx>,
 {
+    // The generator transform relies on the fact that this analysis does **not** use "before"
+    // effects.
+
     fn statement_effect(
         &self,
         trans: &mut impl GenKill<Self::Idx>,

src/librustc_mir/dataflow/impls/init_locals.rs

@@ -33,6 +33,9 @@ impl dataflow::AnalysisDomain<'tcx> for MaybeInitializedLocals {
 }
 
 impl dataflow::GenKillAnalysis<'tcx> for MaybeInitializedLocals {
+    // The generator transform relies on the fact that this analysis does **not** use "before"
+    // effects.
+
     fn statement_effect(
         &self,
         trans: &mut impl GenKill<Self::Idx>,