[MLIR][SparseTensor] Introduce opaque pointers in LLVM dialect lowering (#70570)

This commit changes the SparseTensor LLVM dialect lowering from using
`llvm.ptr<i8>` to `llvm.ptr`. This change ensures that the lowering now
properly relies on opaque pointers, instead of working with already type
erased i8 pointers.

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorConversion.cpp

@@ -42,7 +42,7 @@ namespace {
 /// Maps each sparse tensor type to an opaque pointer.
 static std::optional<Type> convertSparseTensorTypes(Type type) {
   if (getSparseTensorEncoding(type) != nullptr)
-    return LLVM::LLVMPointerType::get(IntegerType::get(type.getContext(), 8));
+    return LLVM::LLVMPointerType::get(type.getContext());
   return std::nullopt;
 }