Suppress errors for unreachable branches in conditional expressions

[brianschubert]

Fixes #4134
Fixes #9195

Suppress errors when analyzing unreachable conditional expression branches. Same idea as what's done when analyzing the right-hand operand of and/or:

mypy/mypy/checkexpr.py

Lines 4252 to 4256 in 973618a

	# If right_map is None then we know mypy considers the right branch
	# to be unreachable and therefore any errors found in the right branch
	# should be suppressed.
	with self.msg.filter_errors(filter_errors=right_map is None):
	right_type = self.analyze_cond_branch(right_map, e.right, expanded_left_type)

This PR originally added filters of the same form to the places where analyze_cond_branch is called in ExpressionChecker.visit_conditional_expr. However, since 5 out of the 6 analyze_cond_branch call sites now use filter_errors for the case when map is None, I decided to move the error filtering logic to inside analyze_cond_branch.

Given:

from typing import TypeVar
T = TypeVar("T", int, str)

def foo(x: T) -> T:
    return x + 1 if isinstance(x, int) else x + "a"

Before:

main.py:5:16: error: Unsupported operand types for + ("str" and "int")  [operator]
main.py:5:49: error: Unsupported operand types for + ("int" and "str")  [operator]
Found 2 errors in 1 file (checked 1 source file)

After:

Success: no issues found in 1 source file

[github-actions]

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

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/_internal/concurrency/cancellation.py:240: error: Unsupported operand types for - ("float" and "None")  [operator]

vision (https://github.com/pytorch/vision)
+ torchvision/transforms/v2/functional/_geometry.py:628: error: Unused "type: ignore" comment  [unused-ignore]

jax (https://github.com/google/jax)
+ jax/_src/numpy/linalg.py:393: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/export/shape_poly.py:1642: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/export/shape_poly.py:1661: error: Unused "type: ignore" comment  [unused-ignore]

graphql-core (https://github.com/graphql-python/graphql-core)
+ src/graphql/type/definition.py:793: error: Unused "type: ignore" comment  [unused-ignore]
+ src/graphql/type/definition.py:898: error: Unused "type: ignore" comment  [unused-ignore]
+ src/graphql/type/definition.py:1365: error: Unused "type: ignore" comment  [unused-ignore]

[JukkaL]

Nice, thanks for the fix!