Adjusting the Stdlib to cope with our conceptual TC problem

[sbscholz]

At the time being (Feb 2026), the default-element generation for Tensor Comprehensions (TCs) does not (yet) follow the translation described in https://dl.acm.org/doi/10.1145/3412932.3412947. Instead, we do the following:

{ iv -> expr(iv) | iv < shp } is translated into

tmp = expr([0,..., 0]);
with {
   (  iv  ) : express(iv);
} : genarray (shp, genarray( shape(tmp), zero(tmp)));

This can lead to errors if the compiler finds the (potentially) illegal selection before optimising it away....
Fortunately, there is a manual work-around. If we write

{ iv -> expr(iv) | iv < shp;
   iv -> const.   | iv < shp};

the compiler will generate

with {
   (  iv  ) : express(iv);
} : genarray (shp, const);

This PR changes (hopefully all) such cases from the former into the latter.

See https://gitlab.sac-home.org/sac-group/sac2c/-/issues/2544 for a detailed description of the compiler issue.