[RS4GC] Use AssertingVH for RematerializedValueMapTy, NFCI

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

 // In the actual implementation this caches two relations:
 // - The base relation itself (i.e. this pointer is based on that one)
 // - The base defining value relation (i.e. before base_phi insertion)
 // Generally, after the execution of a full findBasePointer call, only the
 // base relation will remain.  Internally, we add a mixture of the two
 // types, then update all the second type to the first type
 typedef DenseMap<Value *, Value *> DefiningValueMapTy;
 typedef DenseSet<Value *> StatepointLiveSetTy;
-typedef DenseMap<Instruction *, Value *> RematerializedValueMapTy;
+typedef DenseMap<AssertingVH<Instruction>, AssertingVH<Value>>
+  RematerializedValueMapTy;
 
 struct PartiallyConstructedSafepointRecord {
   /// The set of values known to be live across this safepoint
   StatepointLiveSetTy LiveSet;
 
   /// Mapping from live pointers to a base-defining-value
   DenseMap<Value *, Value *> PointerToBase;