[llvm] [SandboxVec][Interval] Implement getUnionInterval() and getSingleDiff() (PR #111455)

[via llvm-commits]

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@@ -176,6 +176,26 @@ template <typename T> class Interval {
       Result.emplace_back(Intersection.To->getNextNode(), To);
     return Result;
   }
+  /// A difference that asserts that the result is a single interval.
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vporpo wrote:

I will change it to:
`/// \Returns the interval difference `this - Other`. This will crash in the Debug build if the result is not a single interval.`
Wdyt ? 

https://github.com/llvm/llvm-project/pull/111455