reduce: Add two branch reduction passes (#2507)

* Fix #2320. `conditional_branch_to_simple_conditional_branch` reduction pass changes conditional branches so both targets point to the same block id (creating a "simple" conditional branch).
* Fix #2501. `simple_conditional_branch_to_branch` reduction pass changes "simple" conditional branches to branches. 
* Fix #2503. `conditional_branch_to_simple_conditional_branch` proper handling of back-edges.