[Hexagon] Avoid infinite loops in HexagonLoopIdiomRecognition

[Hexagon] Avoid infinite loops in HexagonLoopIdiomRecognition

- Avoid explosive growth of the simplification queue by not queuing
  expressions that are alredy in it.
- Add an iteration counter and abort after a sufficiently large number
  of iterations (assuming that it's a symptom of an infinite loop).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298655 91177308-0d34-0410-b5e6-96231b3b80d8