Fix downcasts of unaligned empty/tombstone DenseMap keys for DenseMap<AssertVH<T>, Foo>.

[samsonov]

Implement a limited fix to the problem, suggested by Richard Smith:
redefine DenseMapInfo<AssertingVH<T>>::isEqual to ensure it doesn't
perform pointer downcasts: fake pointer values for empty/tombstone keys
may not be properly aligned.

[samsonov]

(ping)

[rsmith]

LGTM

include/llvm/IR/ValueHandle.h

 /// AssertingVH's as it moves.  This is required because in non-assert mode this
 /// class turns into a trivial wrapper around a pointer.
 template <typename ValueTy>
 class AssertingVH
 #ifndef NDEBUG
   : public ValueHandleBase
 #endif
   {
+  friend struct DenseMapInfo<AssertingVH<ValueTy> >;
 
 #ifndef NDEBUG
   ValueTy *getValPtr() const {
     return static_cast<ValueTy*>(ValueHandleBase::getValPtr());
   }
   void setValPtr(ValueTy *P) {
     ValueHandleBase::operator=(GetAsValue(P));
   }
static inline AssertingVH<T> getEmptyKey() {
     return AssertingVH<T>(PointerInfo::getEmptyKey());
   }
   static inline T* getTombstoneKey() {
     return AssertingVH<T>(PointerInfo::getTombstoneKey());
   }
   static unsigned getHashValue(const AssertingVH<T> &Val) {
     return PointerInfo::getHashValue(Val);
   }
+#ifndef NDEBUG
+  static bool isEqual(const AssertingVH<T> &LHS, const AssertingVH<T> &RHS) {
+    // Avoid downcasting AssertingVH<T> to T*, as empty/tombstone keys may not
+    // be properly aligned pointers to T*.
+    return LHS.ValueHandleBase::getValPtr() == RHS.ValueHandleBase::getValPtr();
+  }
+#else
   static bool isEqual(const AssertingVH<T> &LHS, const AssertingVH<T> &RHS) {
     return LHS == RHS;
   }
+#endif
 };
-  
 template <typename T>
 struct isPodLike<AssertingVH<T> > {
 #ifdef NDEBUG
   static const bool value = true;
 #else
   static const bool value = false;
 #endif
 };