fix: do not narrow expression type to TypeVar on equality comparison

[ordinary9843]

When a variable is compared with == against a TypeVar operand, mypy was narrowing the left-hand variable to the TypeVar type. This caused false errors on subsequent operations — for example, x: int would be narrowed to T after assert x == y (where y: T), causing x += 1 to be flagged as unsupported operands.

The regression was introduced in #20602, which routed == through narrow_type_by_identity_equality but did not guard against TypeVar targets. A TypeVar represents an unknown concrete type at the call site, so narrowing based on it can only lose information, not gain it.

This fix skips narrowing in narrow_type_by_identity_equality when the target type is a TypeVarLikeType. A more precise approach — intersecting the expression type with the TypeVar's upper bound — would require intersection type support and can be done as a follow-up.

Fixes #21199

[github-actions]

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

zulip (https://github.com/zulip/zulip)
- zerver/lib/validator.py:331: error: Redundant cast to "ResultT"  [redundant-cast]