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

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.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D119636

Author: slydiman

llvm/lib/IR/Value.cpp

@@ -377,6 +377,7 @@ void Value::setName(const Twine &NewName) {
 }
 
 void Value::takeName(Value *V) {
+  assert(V != this && "Illegal call to this->takeName(this)!");
   ValueSymbolTable *ST = nullptr;
   // If this value has a name, drop it.
   if (hasName()) {