[LoopSimplify] Simplify how we compute UniqueExit

[trentxintong]

Summary

Simplify how we compute UniqueExit. Reuse ExitBlockSet.

[efriedma]

LGTM

[trentxintong]

Thanks for taking a quick look. @efriedma !

lib/Transforms/Utils/LoopSimplify.cpp

ReprocessLoop:
   // block, attempt to merge the exits. This helps several passes, such
   // as LoopRotation, which do not support loops with multiple exits.
   // SimplifyCFG also does this (and this code uses the same utility
   // function), however this code is loop-aware, where SimplifyCFG is
   // not. That gives it the advantage of being able to hoist
   // loop-invariant instructions out of the way to open up more
   // opportunities, and the disadvantage of having the responsibility
   // to preserve dominator information.
-  bool UniqueExit = true;
-  if (!ExitBlocks.empty())
-    for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
-      if (ExitBlocks[i] != ExitBlocks[0]) {
-        UniqueExit = false;
-        break;
-      }
-  if (UniqueExit) {
+  if (ExitBlockSet.size() == 1) {
     for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
       BasicBlock *ExitingBlock = ExitingBlocks[i];
       if (!ExitingBlock->getSinglePredecessor()) continue;
       BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
       if (!BI || !BI->isConditional()) continue;
       CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
       if (!CI || CI->getParent() != ExitingBlock) continue;