Rollup merge of #160995 - lqd:readabilly, r=jackh726

Tiny readability cleanups for initialization dataflow analyses

While reading over borrowck liveness, I noticed this handful of small things in the dataflow initialization module.

They're easier to see per-commit but:
- each analysis code was hard to read and locate, as the impls were scattered over the file, so this regroups them
- some of these didn't follow the pattern others did
- we were asserting things we didn't use, and which were already guaranteed by the function we were calling

r? @jackh726

Author: jhpratt

compiler/rustc_mir_dataflow/src/drop_flag_effects.rs

@@ -87,6 +87,8 @@ pub fn on_all_children_bits<'tcx, F>(
     on_all_children_bits(move_data, move_path_index, &mut each_child);
 }
 
+/// Calls `callback` for each child move path of the function's arguments. Note the move paths'
+/// `DropFlagState` argument to the callback will always be `DropFlagState::Present`.
 pub fn drop_flag_effects_for_function_entry<'tcx, F>(
     body: &Body<'tcx>,
     move_data: &MoveData<'tcx>,