Fixed todo's related to mul_en/div_en i EX stage

Created by: silabs-oysteink

Factored halt_ex/kill_ex out of mul_en and div_en - multiplier and divider now handles them theirselves. In the divider, div_en_i and valid_i was then duplicates, changed to using only valid_i.

SEC clean (with multiplier operation converted to add to enable converging SEC)