[LoopIdiom] Support 'left-shift until zero' idiom

This adds support for the "count active bits" pattern, i.e.:
```
int countBits(unsigned val) {
    int cnt = 0;
    for( ; (val << cnt) != 0; ++cnt)
        ;
    return cnt;
}
```
but a somewhat more general one:
```
int countBits(unsigned val, int start, int off) {
    int cnt;
    for (cnt = start; val << (cnt + off); cnt++)
        ;
    return cnt;
}
```

alive2 is happy with all the tests there.

Note that, again, much like with the right-shift cases,
we don't require the `val != 0` guard.

This is the last pattern that was supported by
`detectShiftUntilZeroIdiom()`, which now becomes obsolete.