pp_entersub(): eliminate a label

pp_entersub(): eliminate a label

replace:

  retry:
    if (A) die;
    if (B) {
        ...;
        goto retry;
    }

with

    while (B) {
        ...;
    }

    if (A) die;

it's functionally equivalent except that the A test is now only
tried after we have been successful at B. This is ok, because
A is testing for an uncloned closure prototype, while B is looking
for a CV stub which needs autoloading,and it doesn't rally matter
which we test for first.