Reapplying r136844.

An algorithm for incrementally updating LoopInfo within a
LoopPassManager. The incremental update should be extremely cheap in
most cases and can be used in places where it's not feasible to
regenerate the entire loop forest.

- "Unloop" is a node in the loop tree whose last backedge has been removed.
- Perform reverse dataflow on the block inside Unloop to propagate the
  nearest loop from the block's successors.
- For reducible CFG, each block in unloop is visited exactly
  once. This is because unloop no longer has a backedge and blocks
  within subloops don't change parents.
- Immediate subloops are summarized by the nearest loop reachable from
  their exits or exits within nested subloops.
- At completion the unloop blocks each have a new parent loop, and
  each immediate subloop has a new parent.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137276 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/LoopInfo.h

@@ -708,9 +708,13 @@ public:
   /// specified loop.  This should be used by transformations that restructure
   /// the loop hierarchy tree.
   void changeLoopFor(BlockT *BB, LoopT *L) {
-    LoopT *&OldLoop = BBMap[BB];
-    assert(OldLoop && "Block not in a loop yet!");
-    OldLoop = L;
+    if (!L) {
+      typename DenseMap<BlockT *, LoopT *>::iterator I = BBMap.find(BB);
+      if (I != BBMap.end())
+        BBMap.erase(I);
+      return;
+    }
+    BBMap[BB] = L;
   }
 
   /// changeTopLevelLoop - Replace the specified loop in the top-level loops
@@ -1029,6 +1033,12 @@ public:
     LI.removeBlock(BB);
   }
 
+  /// updateUnloop - Update LoopInfo after removing the last backedge from a
+  /// loop--now the "unloop". This updates the loop forest and parent loops for
+  /// each block so that Unloop is no longer referenced, but the caller must
+  /// actually delete the Unloop object.
+  void updateUnloop(Loop *Unloop);
+
   /// replacementPreservesLCSSAForm - Returns true if replacing From with To
   /// everywhere is guaranteed to preserve LCSSA form.
   bool replacementPreservesLCSSAForm(Instruction *From, Value *To) {