[DebugInstrRef] Use DenseMap for ValueToLoc (NFC)

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.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

@@ -272,7 +272,7 @@ public:
     };
 
     // Map of the preferred location for each value.
-    std::map<ValueIDNum, LocIdx> ValueToLoc;
+    DenseMap<ValueIDNum, LocIdx> ValueToLoc;
     ActiveMLocs.reserve(VLocs.size());
     ActiveVLocs.reserve(VLocs.size());
 

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.h

@@ -1082,7 +1082,9 @@ template <> struct DenseMapInfo<ValueIDNum> {
     return ValueIDNum::TombstoneValue;
   }
 
-  static unsigned getHashValue(const ValueIDNum &Val) { return Val.asU64(); }
+  static unsigned getHashValue(const ValueIDNum &Val) {
+    return hash_value(Val.asU64());
+  }
 
   static bool isEqual(const ValueIDNum &A, const ValueIDNum &B) {
     return A == B;