[LoopInfo] Verify BBMap tracks innermost loops for BBs.

[fhahn]

Summary

By checking that none of the child loops contain a BB we make sure BBMap
contains the innermost loop defining BB. This invariant was violated in
LoopInterchange and got caught by this assertion.

[fhahn]

LoopInterchange did that wrong and this additional assertion catches that. The fix for LoopInterchange is at D45970

[efriedma]

LGTM

While you're at it, could you also add a check to compareLoops to make sure the DenseBlockSet is consistent?

[fhahn]

Add check to make sure the blocks sets match. I had to add a function to get a handle for a const blocks sets, what do you think? I would be happy to split this up in 2 patches if necessary.

[efriedma]

LGTM

include/llvm/Analysis/LoopInfoImpl.h

 
 // Verify that blocks are mapped to valid loops.
 #ifndef NDEBUG
   for (auto &Entry : BBMap) {
     const BlockT *BB = Entry.first;
     LoopT *L = Entry.second;
     assert(Loops.count(L) && "orphaned loop");
     assert(L->contains(BB) && "orphaned block");
+    for (LoopT *ChildLoop : *L)
+      assert(!ChildLoop->contains(BB) &&
+             "BBMap should point to the deepest loop containing BB");
   }
 
   // Recompute LoopInfo to verify loops structure.
   LoopInfoBase<BlockT, LoopT> OtherLI;
   OtherLI.analyze(DomTree);
 
   // Build a map we can use to move from our LI to the computed one. This
   // allows us to ignore the particular order in any layer of the loop forest