add some comments explaining how the required_consts stuff fits together

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -209,6 +209,10 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         caller_location: None,
     };
 
+    // It may seem like we should iterate over `required_consts` to ensure they all successfully
+    // evaluate; however, the `MirUsedCollector` already did that during the collection phase of
+    // monomorphization so we don't have to do it again.
+
     fx.per_local_var_debug_info = fx.compute_per_local_var_debug_info(&mut start_bx);
 
     let memory_locals = analyze::non_ssa_locals(&fx);