Function ideal_binary_op_entry 

pub fn ideal_binary_op_entry(
    a: ValueId,
    b: ValueId,
    result: Option<ValueId>,
    exit_layout: &[TargetSlot],
) -> Vec<TargetSlot>

Computes the ideal pre-layout for a binary operation.

For a binary op that consumes [a, b] (a on top, b below) and produces result:

• If we want [result, …rest] after
• We need [a, b, …rest] before (where rest doesn’t contain a or b)