[zhengguangzhuo]

Fixes #22259.

When --allow-redefinition is enabled, narrowing performed inside a statically always-true if True or if TYPE_CHECKING block is lost after the block. The merge logic treated semantically equivalent types as identical and kept the less-narrowed representation, while the unreachable else path could still contribute the pre-if state.

This change preserves the narrowed representation in the new semantics and excludes the unreachable path from the merge. Regression tests cover if True with and without --allow-redefinition, and TYPE_CHECKING with the option enabled.

Verification:

• Focused tests for check-redefine2, check-unreachable-code, and check-isinstance: 387 passed, 1 skipped, 1 xfailed.
• Mypy self-check: 347 source files passed.
• Repository pre-commit hooks on changed files: all applicable hooks passed.
• git diff --check: passed.

This contribution was prepared with assistance from OpenAI Codex.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pwndbg (https://github.com/pwndbg/pwndbg)
- pwndbg/commands/__init__.py:935: error: Unused "type: ignore" comment  [unused-ignore]
- pwndbg/commands/__init__.py: note: In function "OnlyWhenRunning":
- pwndbg/commands/__init__.py:935: error: Item "function" of "Callable[P, T] | None" has no attribute "_pwndbg_only_when_running_allow_core"  [union-attr]
- pwndbg/commands/__init__.py:935: note: Error code "union-attr" not covered by "type: ignore[attr-defined]" comment

scipy-stubs (https://github.com/scipy/scipy-stubs)
- tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports