reduce: StructuredLoopToSelectionReductionOpportunity should handle loops with a continue target that is also the merge of an inner construct

`StructuredLoopToSelectionReductionOpportunity` simplifies loops to selections, but it will give up conservatively if the continue target of the loop happens to also be a merge block of some other construct. E.g.

```
Loop header
Select header
Select merge / Loop continue target <-- we give up if we see this

Loop merge
```

We should handle this.

Locate StructuredLoopToSelectionReductionOpportunity and trace the conservative check for a loop continue target that is also an inner construct's merge block. Reproduce the Loop header, Select header, and shared Select merge scenario described in the issue; done means the loop is reduced to a selection instead of being abandoned.

Written by the indexing model from the issue text.