add another lint to bit_mask to warn on ineffective masks

[llogiq]

Currently, bad_bit_mask ignores masks that do not make the result of the whole expression constant, but don't add any value, e.g. x|0 or x & -1 or even x | 1 > 2 (because the former are always equal to x and latter is equal to x > 2 – they're not wrong per se, but may be misleading.

So let's have BitMask implement a second lint type (that is Warn instead of Deny), which I'd call ineffective_bit_mask.

[lambda-fairy]

If we aren't already, we should warn about identity operations in general like x * 1 or 0 + x.

[llogiq]

That, too. So the lint should probably only warn about bit masks that are not by themselves identity operations, but which are rendered ineffective by the comparison.

Another "identity_op" lint should then check direct identity operations.

[llogiq]

Done.