currently we always evaluate the binary expression, then coerce it
to zero/null with the `ZeroOrNull` instruction, and then emit a separate
jump.
this is fine for non-conditional expressions where we are using the value
itself (e.g. in a SELECT result column), but in conditionals we don't care
about that at all and just want to jump.
so: try to keep the spirit of code reuse, but still have distinct implementations
for conditionals and non-conditionals.
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