Suppose we have some code like this:

x = ...;
if (c) {
  y = x;
}
// (*)

We get a synonym fact between x and y meaning they are guaranteed to be equivalent whenever they are both available. Importantly they are not synonymous at (*) because y is not available there.

Currently if a conditional branch gets flattened we have the problem where the above turns into:

x = ...;
y = c ? x : arbitrary;
// (*)

We evidently cannot regard x and y as being synonymous now.

Perhaps FlattenConditionalBranch should be inapplicable if any instruction in the region being flattened generates an id that is involved in a synonym or equation.