[LoopInfo] Update getExitEdges to accept vector of pairs for non const BasicBlock

[skatkov]

Summary

D63921 requires getExitEdges fills a vector of Edge pairs where
BasicBlocks are not constant.

This is an alternative solution to D64060

[reames]

Commented on the other, handn't realized this was already posted. I strongly prefer this version.

[fhahn]

LGTM, thanks for the alternative approach. It would be great if you could add a sentence about the reasoning (more consistent with the rest of the Loop APIs) in the commit message.

llvm/include/llvm/Analysis/LoopInfo.h

#endif
   /// dedicated exits.
   void getUniqueExitBlocks(SmallVectorImpl<BlockT *> &ExitBlocks) const;
 
   /// If getUniqueExitBlocks would return exactly one block, return that block.
   /// Otherwise return null.
   BlockT *getUniqueExitBlock() const;
 
   /// Edge type.
-  typedef std::pair<const BlockT *, const BlockT *> Edge;
+  typedef std::pair<BlockT *, BlockT *> Edge;
 
   /// Return all pairs of (_inside_block_,_outside_block_).
   void getExitEdges(SmallVectorImpl<Edge> &ExitEdges) const;
 
   /// If there is a preheader for this loop, return it. A loop has a preheader
   /// if there is only one edge to the header of the loop from outside of the
   /// loop. If this is the case, the block branching to the header of the loop
   /// is the preheader node.

llvm/lib/Transforms/Scalar/LoopPredication.cpp

Optional<LoopICmp> LoopPredication::parseLoopLatchICmp() {
   return Result;
 }
 
 
 bool LoopPredication::isLoopProfitableToPredicate() {
   if (SkipProfitabilityChecks || !BPI)
     return true;
 
-  SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 8> ExitEdges;
+  SmallVector<std::pair<BasicBlock *, BasicBlock *>, 8> ExitEdges;
   L->getExitEdges(ExitEdges);
   // If there is only one exiting edge in the loop, it is always profitable to
   // predicate the loop.
   if (ExitEdges.size() == 1)
     return true;
 
   // Calculate the exiting probabilities of all exiting edges from the loop,
   // starting with the LatchExitProbability.