Use a BumpPtrAllocator for Loop objects

Summary:
And now that we no longer have to explicitly free() the Loop instances, we can
(with more ease) use the destructor of LoopBase to do what LoopBase::clear() was
doing.

Reviewers: chandlerc

Subscribers: mehdi_amini, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D38201

llvm-svn: 314375
diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index 5357596..ab8ae73 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -200,7 +200,7 @@
     assert(OriginalBB == OldLoop->getHeader() &&
            "Header should be first in RPO");
 
-    NewLoop = new Loop();
+    NewLoop = LI->AllocateLoop();
     Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
 
     if (NewLoopParent)