[flang] Allow "non-expandable" scalars in single-element contexts

When a scalar expression is not expandable -- i.e., it would have to be
evaluated once and saved in a temporary to avoid changing the semantics
of the program if it were to be evaluated more than once -- it affects
some aspects of folding and expression semantics.  In cases where
scalar expansion would not cause multiple evaluations due to the shape
of the result having but a single element, however, these "non-expandable"
scalar expressions can be safely allowed.

Differential Revision: https://reviews.llvm.org/D134476