[Bug target/122014] (AArch64) Optimize 8-bit and 16-bit popcount as special cases

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122014

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Try doing:
```
unsigned int popcount_8_bp(uint8_t *x) {
    return (unsigned int)__builtin_popcountg(*x);
}
unsigned int popcount_16_bp(uint16_t *x) {
    return (unsigned int)__builtin_popcountg(*x);
}
```

You will see does a decent job.