Migrate away from llvm::ArrayRef(std::nullopt_t) (#7668)

The upstream LLVM has deprecated ArrayRef(std::nullopt_t).  This CL
migrates away from that.

Author: kazutakahirata

include/triton/Dialect/Triton/IR/TritonOps.td

@@ -1275,7 +1275,7 @@ def ReturnOp : TT_Op<"return", [Pure, HasParent<"FuncOp">, /*MemRefsNormalizable
   let arguments = (ins Variadic<AnyType>:$srcs);
 
   let builders = [OpBuilder<(ins), [{
-    build($_builder, $_state, std::nullopt);
+    build($_builder, $_state, mlir::ValueRange());
   }]>];
 
   let assemblyFormat = "attr-dict ($srcs^ `:` type($srcs))?";

lib/Dialect/Triton/IR/Ops.cpp

@@ -1052,7 +1052,7 @@ void FuncOp::build(OpBuilder &builder, OperationState &state, StringRef name,
     return;
   assert(type.getNumInputs() == argAttrs.size());
   call_interface_impl::addArgAndResultAttrs(
-      builder, state, argAttrs, /*resultAttrs=*/std::nullopt,
+      builder, state, argAttrs, /*resultAttrs=*/{},
       getArgAttrsAttrName(state.name), getResAttrsAttrName(state.name));
 }