Use &IndexSlice instead of &IndexVec where possible

All the same reasons as for `[T]`: more general, less pointer chasing, and `&mut IndexSlice` emphasizes that it doesn't change *length*.

compiler/rustc_mir_transform/src/check_alignment.rs

@@ -128,7 +128,7 @@ fn split_block(
 
 fn insert_alignment_check<'tcx>(
     tcx: TyCtxt<'tcx>,
-    local_decls: &mut LocalDecls<'tcx>,
+    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
     block_data: &mut BasicBlockData<'tcx>,
     pointer: Place<'tcx>,
     pointee_ty: Ty<'tcx>,