Packed arrays are likely common in this case, as with array_shift which
already has a similar optimization.
For the following benchmark:
```php
<?php
for ($i = 0; $i < 10000000; $i++) {
$a = [0, 1, 2, 3, 4, 5];
array_unshift($a, -3, -2, -1);
}
```
On an i7-4790:
```
Benchmark 1: ./sapi/cli/php x.php
Time (mean ± σ): 753.8 ms ± 23.8 ms [User: 749.8 ms, System: 2.1 ms]
Range (min … max): 734.3 ms … 818.6 ms 10 runs
Benchmark 2: ./sapi/cli/php_old x.php
Time (mean ± σ): 972.5 ms ± 5.0 ms [User: 968.8 ms, System: 1.4 ms]
Range (min … max): 967.8 ms … 984.3 ms 10 runs
Summary
./sapi/cli/php x.php ran
1.29 ± 0.04 times faster than ./sapi/cli/php_old x.php
```
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