Function combine_stack_layouts 

pub fn combine_stack_layouts(
    layouts: &[BlockStackLayout],
) -> Option<BlockStackLayout>

Computes a common stack layout for multiple incoming edges at a merge point.

The algorithm:

1. Find values that appear in multiple incoming layouts at the same position
2. For values at different positions, choose the position with lowest total shuffle cost
3. Values only needed by one predecessor become “junk slots” for others

Returns None if the layouts are incompatible (too many different values).