Fix issue with signed/unsigned comparison

Fix issue with signed/unsigned comparison

On Mac (maybe on other platforms using clang?) the following set of
comparisons in the emitter were done as unsigned:
```
(val >= 0xFFFFFFFF80000000LL)
```
even though 'val' is signed. This led to assertion failures. On Windows,
the comparisons were signed.

To fix this, cast the constant to the signed `ssize_t` type (to match `val`).

Fixes #18341