Fix issue with niches on signed values

[danielsn]

Description of changes:

Currently, Kani fails to correctly handle niche optimizations when the underlying data-type is signed.
Use unsigned comparisons (as the rustc backend does) to fix this issue.

Resolved issues:

Resolves #1533

Call-outs:

The niche code is confusing, filed #1540 to track

Testing:

• How is this change tested? Additional test.

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.