[mlir] Fix -Wsign-compare in MeshOps.cpp (NFC)

llvm-project/mlir/lib/Dialect/Mesh/IR/MeshOps.cpp:204:25:
 error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int64_t' (aka 'long') [-Werror,-Wsign-compare]
  if (getShape().size() > rank)
      ~~~~~~~~~~~~~~~~~ ^ ~~~~
1 error generated.