Extract mir_opt_level to a method and use Option to be able to know if the value is provided or not

compiler/rustc_session/src/session.rs

@@ -640,6 +640,9 @@ impl Session {
     pub fn binary_dep_depinfo(&self) -> bool {
         self.opts.debugging_opts.binary_dep_depinfo
     }
+    pub fn mir_opt_level(&self) -> usize {
+        self.opts.debugging_opts.mir_opt_level.unwrap_or(1)
+    }
 
     /// Gets the features enabled for the current compilation session.
     /// DO NOT USE THIS METHOD if there is a TyCtxt available, as it circumvents