[llvm] [LLVM] Use {} instead of std::nullopt to initialize empty ArrayRef (PR #109133)

[Jay Foad via llvm-commits]

================
@@ -10428,7 +10428,7 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, SDVTList VTList,
 
 SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL,
                               SDVTList VTList) {
-  return getNode(Opcode, DL, VTList, std::nullopt);
+  return getNode(Opcode, DL, VTList, ArrayRef<SDValue>());
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jayfoad wrote:

Couldn't use `{}` here due to ambiguity between:
```
SDValue getNode(unsigned Opcode, const SDLoc &DL, SDVTList VTList, ArrayRef<SDValue> Ops);
SDValue getNode(unsigned Opcode, const SDLoc &DL, SDVTList VTList, SDValue N);
```

https://github.com/llvm/llvm-project/pull/109133