[NFC][IR] Value: assert this->takeName(this)

[slydiman]

Summary

Need to add an assert about this->takeName(this). This restriction is already documented, so this is just an NFC check.
Without this assertion (as prescribed by original comments for this API), name deletion or down-stream assert failures may occur in other routines: e.g. at the beginning of replaceAllUsesWith() below.

[aeubanks]

nit: I think the added comments aren't necessary, as you said it's already stated in the header file

llvm/lib/IR/Value.cpp

 
 void Value::setName(const Twine &NewName) {
   setNameImpl(NewName);
   if (Function *F = dyn_cast<Function>(this))
     F->recalculateIntrinsicID();
 }
 
 void Value::takeName(Value *V) {
+  // Without this assertion (as prescribed by original comments for this API),
+  // name deletion or down-stream assert failures may occur in other routines:
+  // e.g. at the beginning of replaceAllUsesWith() below.
+  assert(V != this && "Illegal call to this->takeName(this)!");
   ValueSymbolTable *ST = nullptr;
   // If this value has a name, drop it.
   if (hasName()) {
     // Get the symtab this is in.
     if (getSymTab(this, ST)) {
       // We can't set a name on this value, but we need to clear V's name if
       // it has one.
       if (V->hasName()) V->setName("");