Just replacing std::map with DenseMap here is a major regression
-- because this code used an identity hash for ValueIDNum.
Because ValueIDNum is composed of multiple components, it is
important that we use a reasonably good hash function here, so
switch it to hash_value. DenseMapInfo::getHashValue<uint64_t>
would not be sufficient.

This gives a -0.8% geomean improvement on CTMark ReleaseLTO-g.