Function optimize_dce 

pub fn optimize_dce(
    module: &LcnfModule,
    config: &DceConfig,
) -> (LcnfModule, DceStats)

Run the complete DCE + constant propagation pipeline on a module.

The optimizer runs the enabled passes in a fixed-point loop:

1. Constant propagation
2. Copy propagation
3. Known case folding
4. Dead let elimination
5. Unreachable alt elimination

After the fixed point, dead function elimination is applied once (interprocedural).

Returns the optimized module and accumulated statistics.