Move mir_opt_level getter into Options

compiler/rustc_session/src/session.rs

@@ -562,10 +562,7 @@ impl Session {
         self.opts.debugging_opts.binary_dep_depinfo
     }
     pub fn mir_opt_level(&self) -> usize {
-        self.opts
-            .debugging_opts
-            .mir_opt_level
-            .unwrap_or_else(|| if self.opts.optimize != config::OptLevel::No { 2 } else { 1 })
+        self.opts.mir_opt_level()
     }
 
     /// Gets the features enabled for the current compilation session.