implement valtrees as the type-system representation for constant values

compiler/rustc_mir_transform/src/lib.rs

@@ -531,8 +531,10 @@ fn inner_optimized_mir(tcx: TyCtxt<'_>, did: LocalDefId) -> Body<'_> {
         None => {}
         Some(other) => panic!("do not use `optimized_mir` for constants: {:?}", other),
     }
+    debug!("about to call mir_drops_elaborated...");
     let mut body =
         tcx.mir_drops_elaborated_and_const_checked(ty::WithOptConstParam::unknown(did)).steal();
+    debug!("body: {:#?}", body);
     run_optimization_passes(tcx, &mut body);
 
     debug_assert!(!body.has_free_regions(), "Free regions in optimized MIR");