Optimize array_fill_keys()

[ndossche]

Move the refcount update outside of the loop.

For the following benchmark:

$r = range(0, 1000);
$v = new stdClass();

for ($i = 0; $i < 100000; $i++) {
    array_fill_keys($r, $v);
}

On an i7-4790:

Benchmark 1: ./sapi/cli/php_old ../x.php
  Time (mean ± σ):     507.5 ms ±   4.8 ms    [User: 505.1 ms, System: 1.2 ms]
  Range (min … max):   501.2 ms … 518.4 ms    10 runs

Benchmark 2: ./sapi/cli/php ../x.php
  Time (mean ± σ):     479.8 ms ±   3.1 ms    [User: 476.8 ms, System: 1.8 ms]
  Range (min … max):   475.0 ms … 486.7 ms    10 runs

Summary
  ./sapi/cli/php ../x.php ran
    1.06 ± 0.01 times faster than ./sapi/cli/php_old ../x.php

On an i7-1185G7:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     343.9 ms ±   3.1 ms    [User: 341.1 ms, System: 2.3 ms]
  Range (min … max):   337.9 ms … 347.8 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     357.8 ms ±   2.3 ms    [User: 355.7 ms, System: 1.6 ms]
  Range (min … max):   355.0 ms … 362.6 ms    10 runs

Summary
  ./sapi/cli/php x.php ran
    1.04 ± 0.01 times faster than ./sapi/cli/php_old x.php