Disable bf16 tests on AArch64

[awarzynski]

Summary

This patch disables 2 bf16 tests that are currently not supported on
AArch64. I've triaged these failures and opened [1] to track this. I
don't have a simple reproducer for dense_output_bf16.mlir, but it's
rather clear that both tests fail due to missing support for bfloat
operations in the AArch64 backend.

I'm not sure what the path forward to enable these tests on AArch64
should be. I think that there are two options:

• AArch64 backened gains capability to legalize these nodes containing bfloat operands, or
• MLIR (similarly to Clang) is taught not to emit such nodes in the first place.

[1] https://github.com/llvm/llvm-project/issues/58465

mlir/test/Integration/Dialect/SparseTensor/CPU/dense_output_bf16.mlir

 // RUN: mlir-opt %s --sparse-compiler | \
 // RUN: mlir-cpu-runner \
 // RUN:  -e entry -entry-point-result=void  \
 // RUN:  -shared-libs=%mlir_lib_dir/libmlir_c_runner_utils%shlibext | \
 // RUN: FileCheck %s
 
+// UNSUPPORTED: aarch64
+
 #SparseVector = #sparse_tensor.encoding<{dimLevelType = ["compressed"]}>
 #DenseVector = #sparse_tensor.encoding<{dimLevelType = ["dense"]}>
 
 #trait_vec_op = {
   indexing_maps = [
     affine_map<(i) -> (i)>,  // a (in)
     affine_map<(i) -> (i)>,  // b (in)
     affine_map<(i) -> (i)>   // x (out)

mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_sum_bf16.mlir

 // RUN: mlir-opt %s --sparse-compiler | \
 // RUN: mlir-cpu-runner \
 // RUN:  -e entry -entry-point-result=void  \
 // RUN:  -shared-libs=%mlir_lib_dir/libmlir_c_runner_utils%shlibext | \
 // RUN: FileCheck %s
 
+// UNSUPPORTED: aarch64
+
 !Filename = !llvm.ptr<i8>
 
 #SparseMatrix = #sparse_tensor.encoding<{
   dimLevelType = [ "compressed", "compressed" ]
 }>
 
 #trait_sum_reduce = {
   indexing_maps = [