[Bug tree-optimization/90838] Detect table-based ctz implementation

[ktkachov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838

--- Comment #5 from ktkachov at gcc dot gnu.org ---
FWIW, there is another similar function in deepsjeng that computes a
side-effect:
int
myctz2 (unsigned long long * const b) {
     unsigned long long lsb = (*b) & -(*b);
    *b ^= lsb;
    return table[(lsb * magic) >> 58];
}

so we'd need to make sure that our solution handles multiple uses of lsb