Fixes false negative when slice bound uses length of string literal.
We were meant to check the following, for example. Given:
```python
text[:bound] if text.endswith(suffix) else text
```
We want to know whether:
- `suffix` is a string literal and `bound` is a number literal
- `suffix` is an expression and `bound` is
exactly `-len(suffix)` (as AST nodes, prior to evaluation.)
The issue is that negative number literals like `-10` are stored as
unary operators applied to a number literal in the AST. So when `suffix`
was a string literal but `bound` was `-len(suffix)` we were getting
caught in the match arm where `bound` needed to be a number. This is now
fixed with a guard.
Closes#16231
