Skip shim passes if they've already been run

Looks like Generator drop shims already have `post_borrowck_cleanup` run
on them. That's a bit surprising, since it means they're getting const-
and maybe borrow-checked? This merits further investigation, but for now
just preserve the status quo.

compiler/rustc_mir_transform/src/shim.rs

@@ -75,6 +75,14 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceDef<'tcx>) -> Body<'
     };
     debug!("make_shim({:?}) = untransformed {:?}", instance, result);
 
+    // In some of the above cases, we seem to be invoking the passes for non-shim MIR bodies.
+    // If that happens, there's no need to run them again.
+    //
+    // FIXME: Is this intentional?
+    if result.phase >= MirPhase::Const {
+        return result;
+    }
+
     pm::run_passes(
         tcx,
         &mut result,