Better narrows bool in if statements

[sobolevn]

For some reason previously bool was narrowed like this:

y: bool
if y is True:
    reveal_type(y)  # N: Revealed type is "Literal[True]"
else:
    reveal_type(y)  # N: Revealed type is "bool"

Which is not ideal, when we can do better:

y: bool
if y is True:
    reveal_type(y)  # N: Revealed type is "Literal[True]"
else:
    reveal_type(y)  # N: Revealed type is "Literal[False]"

[bluetech]

I wonder how this relates to #6113 (which also points to an open PR #10389)?

[sobolevn]

@bluetech yes, this looks like a very related issue. My approach is quite minimalistic compared to #10389 (which looks like a more full-featured solution).

Let's leave it @JukkaL to decide what to do.

[hauntsaninja]

My hesitation with this PR is that if bool_var is True is a somewhat unidiomatic check (I'd only recommend it for variables of type Optional[bool]), and I suspect solving the more general if bool_var case (as #10389 attempts to), will solve this case for free.