Optimize dict expand check, move allow check after the thresholds check

dictExpandAllowed (for the main db dict and the expire dict) seems to
involve a few function calls and memory accesses, and we can do it only
after the thresholds checks and can get some performance improvements.

A simple benchmark test: there are 11032768 fixed keys in the database,
start a redis-server with `--maxmemory big_number --save ""`,
start a redis-benchmark with `-P 100 -r 1000000000 -t set -n 5000000`,
collect `throughput summary: n requests per second` result.

After five rounds of testing:
```
unstable     this PR
848032.56    897988.56
854408.69    913408.88
858663.94    914076.81
871839.56    916758.31
882612.56    920640.75
```

We can see a 5% performance improvement in general condition.
But note we'll still hit the degradation when we over the thresholds.

src/dict.c

@@ -1410,13 +1410,13 @@ static int _dictExpandIfNeeded(dict *d)
      * table (global setting) or we should avoid it but the ratio between
      * elements/buckets is over the "safe" threshold, we resize doubling
      * the number of buckets. */
-    if (!dictTypeExpandAllowed(d))
-        return DICT_OK;
     if ((dict_can_resize == DICT_RESIZE_ENABLE &&
          d->ht_used[0] >= DICTHT_SIZE(d->ht_size_exp[0])) ||
         (dict_can_resize != DICT_RESIZE_FORBID &&
          d->ht_used[0] / DICTHT_SIZE(d->ht_size_exp[0]) > dict_force_resize_ratio))
     {
+        if (!dictTypeExpandAllowed(d))
+            return DICT_OK;
         return dictExpand(d, d->ht_used[0] + 1);
     }
     return DICT_OK;