Reduce visibilities, and add warn(unreachable_pub).

Lots of unnecessary `pub`s in this crate. Most are downgraded to
`pub(super)`, though some don't need any visibility.

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -55,7 +55,7 @@ use tracing::{debug, instrument, trace};
 
 use crate::cost_checker::CostChecker;
 
-pub struct JumpThreading;
+pub(super) struct JumpThreading;
 
 const MAX_BACKTRACK: usize = 5;
 const MAX_COST: usize = 100;