[IPO] Restore a call to has_value (NFC)

[kazu]

Summary

This patch restores a call to has_value to make it clear that we are
checking the presence of an optional value, not the underlying value.

This patch partially reverts d08f34b592ff06ccb1f36da88ec09aa926427a4d.

[jdoerfert]

LG

llvm/include/llvm/Transforms/IPO/Attributor.h

struct Access {
 
     /// Return the actual instruction that causes the access.
     Instruction *getRemoteInst() const { return RemoteI; }
 
     /// Return true if the value written is not known yet.
     bool isWrittenValueYetUndetermined() const { return !Content; }
 
     /// Return true if the value written cannot be determined at all.
-    bool isWrittenValueUnknown() const { return Content && !*Content; }
+    bool isWrittenValueUnknown() const {
+      return Content.has_value() && !*Content;
+    }
 
     /// Return the type associated with the access, if known.
     Type *getType() const { return Ty; }
 
     /// Return the value writen, if any. As long as
     /// isWrittenValueYetUndetermined return true this function shall not be
     /// called.
     Value *getWrittenValue() const { return *Content; }