tcg/s390x: Mask TCGMemOp appropriately for indexing

Commit 2b7ec66f fixed TCGMemOp masking following the MO_AMASK addition,
but two cases were forgotten in the TCG S390 backend.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>