Add CallBase::getCalledFunctionName

There's a lot of uses of `getCalledFunction()->getName()`, without ever
checking if `getCalledFunction()` returns a nullptr. Add a new helper
that returns an `std::optional<StringRef>`, that can be used to

- Avoid method chaining
- Have a more safe shortcut to get the callee name.

Author: tsymalla

llvm/include/llvm/IR/InstrTypes.h

@@ -1345,6 +1345,16 @@ class CallBase : public Instruction {
     return nullptr;
   }
 
+  /// Shortcut to retrieving the name of the called function.
+  /// Returns std::nullopt, if the function cannot be found.
+  std::optional<StringRef> getCalledFunctionName() const {
+    Function *F = getCalledFunction();
+    if (F)
+      return F->getName();
+
+    return std::nullopt;
+  }
+
   /// Return true if the callsite is an indirect call.
   bool isIndirectCall() const;