[flang][runtime] Fix another IsContiguous edge case (#69199)

A recent PR addressed zero and one element edge cases but did not cover
another case where the descriptors of arrays with more than two elements
may have byte strides that are not perfect multiples, like when creating
a descriptor for A(:, 1:1:2).

In general, the byte stride in a dimension is only meaningful if that
dimension has more than one element. Update IsContiguous and
CFI_is_contiguous to reflect that.