add error pruning on step_first/step_last to avoid use against lookup

[hero78119]

Description

This works

cb.step_first(|cb| {
            cb.require_equal("xxxxx", A, B); // it works!
});

However it doesnt works on any lookup wrap by step, e.g.

cb.step_first(|cb| {
       cb.call_context_lookup_write(Some(call_id.expr()), field_tag, value); // lookup will happen on any step other than step 1
});

The reason is because lookup constraints expression not including location selector when query lookup cell.
https://github.com/privacy-scaling-explorations/zkevm-circuits/blob/main/zkevm-circuits/src/evm_circuit/util/constraint_builder.rs#L1489-L1496
While the location selector are created in execution.rs at later phase

zkevm-circuits/zkevm-circuits/src/evm_circuit/execution.rs

Lines 731 to 757 in 09651d4

	let sel_step: &dyn Fn(&mut VirtualCells<F>) -> Expression<F> =
	&|meta| meta.query_advice(q_step, Rotation::cur());
	let sel_step_first: &dyn Fn(&mut VirtualCells<F>) -> Expression<F> =
	&|meta| meta.query_selector(q_step_first);
	let sel_step_last: &dyn Fn(&mut VirtualCells<F>) -> Expression<F> =
	&|meta| meta.query_selector(q_step_last);
	let sel_not_step_last: &dyn Fn(&mut VirtualCells<F>) -> Expression<F> = &|meta| {
	meta.query_advice(q_step, Rotation::cur()) * not::expr(meta.query_selector(q_step_last))
	};
	for (selector, constraints) in [
	(sel_step, constraints.step),
	(sel_step_first, constraints.step_first),
	(sel_step_last, constraints.step_last),
	(sel_not_step_last, constraints.not_step_last),
	] {
	if !constraints.is_empty() {
	meta.create_gate(name, |meta| {
	let q_usable = meta.query_selector(q_usable);
	let selector = selector(meta);
	constraints.into_iter().map(move |(name, constraint)| {
	(name, q_usable.clone() * selector.clone() * constraint)
	})
	});
	}
	}

when converting constraints to custom gate. This works for normal constraints but not works for lookup expression.

This PR add error pruning step to avoid lookup usage against step_first/step_last

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

[han0110]

LGTM!

[KimiWu123]

LGTM!