Fix intermittent failures on the H8, particularly H8/SX tests.

Fix intermittent failures on the H8, particularly H8/SX tests.

    The upstream GCC tester has  showed spurious execution failures on the
    H8 target for the H8/SX multilibs. I suspected memory corruption or an
    uninitialized variable early as the same binary would sometimes work and
    sometimes it got the wrong result. Worse yet, the point where the test
    determined it was getting the wrong result would change.

    Because it only happened on the H8/SX variant I was able to zero in on
    the "mova" support and the "short form" of those instructions in particular.

    As the code stands it checks if code->op3.type == 0 to try and identify cases
    where op3 wasn't filled in and thus we've got the short form of the mova
    instruction.

    But for the short-form of those instructions we never set any of the "op3"
    data structure. We get whatever was lying around -- it's usually zero and
     thus things usually work, but if the stale data was nonzero, then we'd
    fail to recognize the instruction as a short-form and fail to set up the
    various fields appropriately.

    I initially initialized the op3.type field to zero, but didn't like that
     because it was inconsistent with how other operands were initialized.
    Bringing consistency meant using -1 as the initializer value and adjusting
    the check for short form mova appropriately.

    I've had this in the upstream GCC tester for perhaps a year at this point
    and haven't seen any of the intermittent failures again.