[mlir] Fix build failure in MeshShardingInterfaceImpl.cpp (NFC)

llvm-project/mlir/lib/Dialect/Linalg/Transforms/MeshShardingInterfaceImpl.cpp:96:8:
error: unused variable 'resultElementType' [-Werror,-Wunused-variable]
  Type resultElementType =
       ^
llvm-project/mlir/lib/Dialect/Linalg/Transforms/MeshShardingInterfaceImpl.cpp:122:1:
error: non-void function does not return a value in all control paths [-Werror,-Wreturn-type]
}
^
2 errors generated.