[LoopIdiom] Support 'arithmetic right-shift until zero' idiom

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

This directly matches the existing 'logical right-shift until zero' idiom.
alive2 is happy with all the tests there.

Note that, again, much like with the original unsigned case,
we don't require the `val != 0` guard.

The old `detectShiftUntilZeroIdiom()` already supports this pattern,
the idea here is that the `val` must be positive (have at least one
leading zero), because otherwise the loop is non-terminating,
but since it is not `while(1)`, that would have been UB.