Add a fast path for setName("") on an unnamed value.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79492 91177308-0d34-0410-b5e6-96231b3b80d8

lib/VMCore/Value.cpp

@@ -171,6 +171,10 @@ std::string Value::getNameStr() const {
 }
 
 void Value::setName(const Twine &NewName) {
+  // Fast path for common IRBuilder case of setName("") when there is no name.
+  if (NewName.isTriviallyEmpty() && !hasName())
+    return;
+
   SmallString<256> NameData;
   NewName.toVector(NameData);