Fix failing assertion in witgen

[georgwiese]

When writing #2119, I came across another issue in witgen. When running this file on main:

namespace main(4);

    // Two bit-constrained witness columns
    // In practice, bit1 is always one (but this is not constrained)
    col witness bit1(i) query Query::Hint(1);
    col witness bit2;
    bit1 * (bit1 - 1) = 0;
    bit2 * (bit2 - 1) = 0;

    // Constrain their sum to be binary as well.
    // This ensures that at most one of the two bits is set.
    // Therefore, bit2 is always zero.
    let bit_sum;
    bit_sum = bit1 + bit2;
    bit_sum * (bit_sum - 1) = 0;

    // Some witness that depends on bit2.
    col witness foo;
    foo = bit2 * 42 + (1 - bit2) * 43;

I'm getting

thread 'main' panicked at /Users/georg/coding/powdr/executor/src/witgen/global_constraints.rs:258:17:
assertion failed: known_constraints.insert(p, RangeConstraint::from_max_bit(0)).is_none()

This is because when it sees the binary range constraint bit_sum * (bit_sum - 1) = 0, it already figured out that bit_sum can be at most 1 because of bit_sum = bit1 + bit2.

This PR fixes it.