$$ is different in block versus in &requires

[evantypanski]

Found via bbannier/spicy-lint#54

Given this program:

module Test;

public type Data = unit {
    x: uint8 &convert=($$ + 5) &requires=($$ < 50) {
        print $$ < 50;
    }
};

I would expect in to print True every time since the &requires succeeded, but instead it may print False:

$ echo '0' | spicy-driver test.spicy
False

I think $$ should continue to refer to the pre-conversion value, personally, but instead it refers to the pre-conversion value in &requires, but after-conversion value in the block.