PHP array_shift(): Optimize tail move on hole-free packed arrays

[mehmetcansahin]

For a packed array without holes, removing the first element leaves a contiguous tail. When no iterator is active, array_shift() now moves that tail with a single memmove() instead of testing every slot for IS_UNDEF; a one element tail is moved directly to avoid the call overhead. Packed arrays with holes, arrays with active iterators, and hash arrays keep the existing paths.

Benchmarks

Draining a packed int array ($a = $base; while ($a) array_shift($a);):

n	repetitions	base	this PR	speedup
100	20,000	160.5 ms	58.7 ms	2.7×
1,000	2,000	1,075.6 ms	249.9 ms	4.3×
10,000	200	10,726.5 ms	2,602.4 ms	4.1×

Two-element regression check ($a = [0, 1]; array_shift($a); x 100M, exercises the direct-copy specialization): 3.528s → 3.438 s, no tiny-array slowdown observed.

Benchmark scripts

Draining benchmark:

<?php
foreach ([100, 1000, 10000] as $n) {
    $reps = intdiv(2000000, $n);
    $base = range(0, $n - 1);
    $best = PHP_FLOAT_MAX;
    for ($run = 0; $run < 3; $run++) {
        $t = hrtime(true);
        for ($r = 0; $r < $reps; $r++) {
            $a = $base;
            while ($a) array_shift($a);
        }
        $best = min($best, (hrtime(true) - $t) / 1e6);
    }
    printf("n=%-6d drain x%-5d best: %8.1f ms\n", $n, $reps, $best);
}

Two-element regression check:

<?php
$best = PHP_FLOAT_MAX;
for ($run = 0; $run < 3; $run++) {
    $t = hrtime(true);
    for ($i = 0; $i < 100000000; $i++) {
        $a = [0, 1];
        array_shift($a);
    }
    $best = min($best, (hrtime(true) - $t) / 1e9);
}
printf("fresh [0,1] x100M best: %.3f s\n", $best);