Avoids the use of "using" in favour of explicit qualification, to be
consistent with spirv-fuzz. Fixes indentation in a TODO comment.
Addresses and removes two existing TODO comments by moving some helper
functionality into reduction_util.
Related issue: #2184.
* 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.
* Add OperandToUndefReductionPass.
Fixes#2115.
Also added some tests that are similar to those in OperandToConstantReductionPassTest.
In addition, refactor FindOrCreateGlobalUndef into reduction_util.cpp. Fixes#2184.
Removed many documentation comments that were identical or very similar to the overridden function's documentation comment.
