Function emit_case 

pub fn emit_case(
    ctx: &mut EmitContext,
    sess: &mut EmitSession<'_>,
    builder: &mut FunctionBuilder<'_>,
    scrut: SsaVal,
    binder: &VarId,
    alts: &[Alt<usize>],
    tail: TailCtx,
) -> Result<SsaVal, EmitError>

Emit Case dispatch. The scrutinee has already been evaluated (stack-safe).