Optimize SplFixedArray::fromArray() for packed arrays

[nielsdos]

If the array is packed, then we don't have to loop to get the highest index.

For this script:

$array = range(1, 100);
for ($i=0;$i<1000000;$i++) {
    SplFixedArray::fromArray($array);
}

On an i7-4790:

Benchmark 1: ./sapi/cli/php spl.php
  Time (mean ± σ):     376.5 ms ±   2.0 ms    [User: 372.1 ms, System: 2.6 ms]
  Range (min … max):   373.7 ms … 379.5 ms    10 runs

Benchmark 2: ./sapi/cli/php_old spl.php
  Time (mean ± σ):     511.6 ms ±   1.9 ms    [User: 508.0 ms, System: 2.3 ms]
  Range (min … max):   509.2 ms … 515.1 ms    10 runs

Summary
  ./sapi/cli/php spl.php  ran
    1.36 ± 0.01 times faster than ./sapi/cli/php_old spl.php

On an i7-1185G7:

Benchmark 1: ./sapi/cli/php spl.php
  Time (mean ± σ):     250.4 ms ±   3.5 ms    [User: 246.6 ms, System: 2.6 ms]
  Range (min … max):   247.0 ms … 258.5 ms    11 runs

Benchmark 2: ./sapi/cli/php_old spl.php
  Time (mean ± σ):     328.4 ms ±   1.0 ms    [User: 324.4 ms, System: 3.8 ms]
  Range (min … max):   327.5 ms … 331.0 ms    10 runs

Summary
  ./sapi/cli/php spl.php  ran
    1.31 ± 0.02 times faster than ./sapi/cli/php_old spl.php

Bonus: this also decreases the code size of the function.