nock: simple branch elimination#850
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I realized that branch elimination introduced in #850 led to crashes like in this example: ``` .*(0 [%6 [%5 [%0 1] %1 0] [%1 42] [%6 [%1 %0] [%0 0 0] [%1 42]]]) ``` When outer %6 is compiled, `_n_formulaic` checks the validity of formulas in branches, replacing them with single `BAIL` instruction if the formula is malformed, which would make it always crash. From POV of _n_formulaic inner %6 is valid, since the condition expression and at least one of the branch expressions are valid. But when inner %6 is compiled, we only compile the invalid branch due to branch elimination, leading to a crash. For that reason _n_formulaic has to be more strict in the presence of branch elimination.
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Adds simple branch elimination during nock compilation.
Together with urbit/urbit#7203,
~?is compiled to just three ops whenverbis set to|, and there are no branching instructions if it is set to either&or|: