[DebugInfo][InstrRef][NFC] Add a missing assignment operator

ValueIDNum is supposed to be a value type that boils down to a uint64_t,
that has some bitfields for convenience. If we use the default operator=,
we end up with each bit field being individually assigned, which is
un-necessarily slow.

Implement the assignment operator by just copying the uint64_t value of
the object. This is quicker, and matches how the comparison operators
work already. Doing so is 0.1% faster on the compile-time-tracker.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.h

@@ -132,6 +132,11 @@ public:
     u.s = {Block, Inst, Loc.asU64()};
   }
 
+  ValueIDNum &operator=(const ValueIDNum &Other) {
+    u.Value = Other.u.Value;
+    return *this;
+  }
+
   uint64_t getBlock() const { return u.s.BlockNo; }
   uint64_t getInst() const { return u.s.InstNo; }
   uint64_t getLoc() const { return u.s.LocNo; }