[mypyc] Add 'bit' primitive type and streamline branching (#9606)
Add the `bit` primitive type that only has two valid values: 0 and 1. The existing
boolean primitive type is different as it also supports a third value (for error).
This makes boolean a poor fit for low-level IR operations which never raise an
exception.
Use `bit` as the result type of comparison operations and various primitives.
Also simplify the branch operation to not have a special mode for branching on
negative values, since it's needlessly ad hoc. Primitives still support this mode,
but it gets converted away during IR building.
Work on mypyc/mypyc#748.
