[mlir] Fixed dynamic operand storage on big-endian machines.

Many tests fails by D101969 (https://reviews.llvm.org/D101969

)
on big-endian machines. This patch changes bit order of
TrailingOperandStorage in big-endian machines. This patch
works on System Z (Triple = "s390x-ibm-linux", CPU = "z14").

Signed-off-by: Haruki Imai <imaihal@jp.ibm.com>

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D104225