There are situations where the arguments of intrinsics must be conformable,
which is defined in section 3.36. This means they must have "the same shape,
or one being an array and the other being scalar". But the check we were
actually making was that their ranks were the same.
This change fixes that and adds a test for the UNPACK intrinsic, where the
FIELD argument "shall be conformable with MASK".
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