Wrong type in `__count_leading_zeros_varying_i64` in `generic.ispc`

While implementing a new feature, I just found out that the function `__count_leading_zeros_varying_i64` in `generic.ispc` internally store the result into a 32-bit integer variable while the function returns a 64-bit type. Do you agree? If so, should the function return a 32-bit int or the result be a 64-bit one? Considering the function in the doc, I would say that this is the former (32-bit return type).

The same thing likely also applies on `__count_trailing_zeros_varying_i64` and the uniform versions (`__count_trailing_zeros_uniform_i64` and `__count_leading_zeros_uniform_i64`). What about using an explicit cast like in `__popcnt_xxx`?

Besides, in the documentation, the output type is not clear for 64-bit types since it is not explicitly reported:

```
A few functions determine how many leading bits in the given value are zero and how many of the trailing bits are zero;
there are also unsigned variants of these functions and variants that take int64 and unsigned int64 types.

int32 count_leading_zeros(int32 v)
uniform int32 count_leading_zeros(uniform int32 v)
int32 count_trailing_zeros(int32 v)
uniform int32 count_trailing_zeros(uniform int32 v)
```