target/arm: Fix MemOp for STGP

When converting to decodetree, the code to rebuild mop for the pair
only made it into trans_STP and not into trans_STGP.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1790


Fixes: 8c212eb6 ("target/arm: Convert load/store-pair to decodetree")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230726165416.309624-1-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>