reduce: revisit later: simplification of conditional branches and selection constructs

Current plan:
1. Simplify conditional branches by making both branch targets the same.
2. Simplify conditional branches with one target (and with no OpSelectionMerge) to just an OpBranch.
3. Simplify selections with a simplified conditional branch by removing the OpSelectionMerge instruction, where possible.

We may want to change this. E.g. 3. could also easily change the conditional branch to an OpBranch.

No files, tests, or entry points are named. Start by reviewing the three-step plan and the proposed alternative for simplifying conditional branches, then clarify which behavior should be adopted; done means an agreed simplification strategy with its scope defined.

Written by the indexing model from the issue text.