Add asan hooks to Recycler and ArrayRecycler

[nhaehnle]

Summary

Already fix some use-after-frees.

[nhaehnle]

Extract the MergeStore part to D25601.

include/llvm/Support/ArrayRecycler.h

class ArrayRecycler {
   // Remove an entry from the free list in Bucket[Idx] and return it.
   // Return NULL if no entries are available.
   T *pop(unsigned Idx) {
     if (Idx >= Bucket.size())
       return nullptr;
     FreeList *Entry = Bucket[Idx];
     if (!Entry)
       return nullptr;
+    __asan_unpoison_memory_region(Entry,
+                                  sizeof(T) * Capacity::get(Idx).getSize());
     Bucket[Idx] = Entry->Next;
     return reinterpret_cast<T*>(Entry);
   }
 
   // Add an entry to the free list at Bucket[Idx].
   void push(unsigned Idx, T *Ptr) {
     assert(Ptr && "Cannot recycle NULL pointer");
     FreeList *Entry = reinterpret_cast<FreeList*>(Ptr);
     if (Idx >= Bucket.size())
       Bucket.resize(size_t(Idx) + 1);
     Entry->Next = Bucket[Idx];
     Bucket[Idx] = Entry;
+    __asan_poison_memory_region(Entry,
+                                sizeof(T) * Capacity::get(Idx).getSize());
   }
 
 public:
   /// The size of an allocated array is represented by a Capacity instance.
   ///
   /// This class is much smaller than a size_t, and it provides methods to work
   /// with the set of legal array capacities.
   class Capacity {

include/llvm/Support/Recycler.h

struct FreeNode {
     FreeNode *Next;
   };
 
   /// List of nodes that have deleted contents and are not in active use.
   FreeNode *FreeList = nullptr;
 
   FreeNode *pop_val() {
     auto *Val = FreeList;
+    // Always unpoison the full Size, because the combiner relies on being able
+    // to morph SDNodes into MachineSDNodes.
+    __asan_unpoison_memory_region(FreeList, Size);
     FreeList = FreeList->Next;
     return Val;
   }
 
   void push(FreeNode *N) {
     N->Next = FreeList;
     FreeList = N;
+    __asan_poison_memory_region(FreeList, Size);
   }
 
 public:
   ~Recycler() {
     // If this fails, either the callee has lost track of some allocation,
     // or the callee isn't tracking allocations and should just call
     // clear() before deleting the Recycler.
     assert(!FreeList && "Non-empty recycler deleted!");