Make MIR basic blocks field public

This makes it possible to mutably borrow different fields of the MIR
body without resorting to methods like `basic_blocks_local_decls_mut_and_var_debug_info`.

To preserve validity of control flow graph caches in the presence of
modifications, a new struct `BasicBlocks` wraps together basic blocks
and control flow graph caches.

The `BasicBlocks` dereferences to `IndexVec<BasicBlock, BasicBlockData>`.
On the other hand a mutable access requires explicit `as_mut()` call.

compiler/rustc_mir_transform/src/lower_slice_len.rs

@@ -27,12 +27,10 @@ pub fn lower_slice_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     };
 
     // The one successor remains unchanged, so no need to invalidate
-    let (basic_blocks, local_decls, _) =
-        body.basic_blocks_local_decls_mut_and_var_debug_info_no_invalidate();
-
+    let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
     for block in basic_blocks {
         // lower `<[_]>::len` calls
-        lower_slice_len_call(tcx, block, &*local_decls, slice_len_fn_item_def_id);
+        lower_slice_len_call(tcx, block, &body.local_decls, slice_len_fn_item_def_id);
     }
 }