[IR] improve hashing quality for ValueInfo

The current hashing quality for ValueInfo is poor because it uses
pointers as the hash value, which can negatively impact performance in
various places that use a DenseSet/Map of ValueInfo. In one observed
case, ModuleSummaryIndex::propagateAttributes() was taking about 25
minutes to complete on a ThinLTO application. Profiling revealed that
the majority of this time was spent operating on the
MarkedNonReadWriteOnly set.

With the improved hashing, the execution time for propagateAttributes is
dramatically reduced to less than 10 seconds.

Author: nocchijiang

llvm/include/llvm/IR/ModuleSummaryIndex.h

@@ -300,7 +300,7 @@ template <> struct DenseMapInfo<ValueInfo> {
     assert(isSpecialKey(L) || isSpecialKey(R) || (L.haveGVs() == R.haveGVs()));
     return L.getRef() == R.getRef();
   }
-  static unsigned getHashValue(ValueInfo I) { return (uintptr_t)I.getRef(); }
+  static unsigned getHashValue(ValueInfo I) { return hash_value(I.getRef()); }
 };
 
 // For optional hinted size reporting, holds a pair of the full stack id