[mlir] Fix warnings

This patch fixes:

  mlir/lib/Dialect/XeGPU/Transforms/XeGPUSubgroupDistribute.cpp:54:3:
  error: definition of implicit copy assignment operator for 'Layout'
  is deprecated because it has a user-declared copy constructor
  [-Werror,-Wdeprecated-copy]

  mlir/lib/Dialect/XeGPU/Transforms/XeGPUSubgroupDistribute.cpp:103:3:
  error: definition of implicit copy assignment operator for 'SGMap'
  is deprecated because it has a user-declared copy constructor
  [-Werror,-Wdeprecated-copy]

Author: kazutakahirata

mlir/lib/Dialect/XeGPU/Transforms/XeGPUSubgroupDistribute.cpp

@@ -51,7 +51,6 @@ namespace {
 struct Layout {
   SmallVector<int64_t, 3> layout;
   Layout() = default;
-  Layout(const Layout &other) = default;
   Layout(std::initializer_list<int64_t> list) : layout(list) {}
   void print(llvm::raw_ostream &os) const;
   size_t size() const { return layout.size(); }
@@ -100,7 +99,6 @@ struct SGMap {
 
 public:
   SGMap() = default;
-  SGMap(const SGMap &other) = default;
   SGMap(const WiLayout &layout, const WiData &data)
       : wiLayout(layout), wiData(data) {}