[red-knot] Fix Leaking Narrowing Constraint in ast::ExprIf (#14590)

## Summary

Closes #14588


```py
x: Literal[42, "hello"] = 42 if bool_instance() else "hello"
reveal_type(x)  # revealed: Literal[42] | Literal["hello"]

_ = ... if isinstance(x, str) else ...

# The `isinstance` test incorrectly narrows the type of `x`.
# As a result, `x` is revealed as Literal["hello"], but it should remain Literal[42, "hello"].
reveal_type(x)  # revealed: Literal["hello"]
```

## Test Plan
mdtest included!

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/red_knot_python_semantic/resources/mdtest/expression/if.md

@@ -22,3 +22,22 @@ reveal_type(1 if None else 2)  # revealed: Literal[2]
 reveal_type(1 if "" else 2)  # revealed: Literal[2]
 reveal_type(1 if 0 else 2)  # revealed: Literal[2]
 ```
+
+## Leaked Narrowing Constraint
+
+(issue #14588)
+
+The test inside an if expression should not affect code outside of the block.
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x: Literal[42, "hello"] = 42 if bool_instance() else "hello"
+
+reveal_type(x)  # revealed: Literal[42] | Literal["hello"]
+
+_ = ... if isinstance(x, str) else ...
+
+reveal_type(x)  # revealed: Literal[42] | Literal["hello"]
+```