fortran: Fix ICE with loops with (deprecated) non-integral iterators [PR126303]

do_subscript contains
              do_sym = dl->ext.iterator->var->symtree->n.sym;
              if (do_sym->ts.type != BT_INTEGER)
                continue;
check to ignore loops with non-integral iterator.  But r15-11154
PR94978 change has added inner_loop_may_be_skipped function and that
happily accesses inner loop iterator's step, start and end if EXPR_CONSTANT
with mpz_init_set (..., ...value.integer);
Now, given the layout of mpfr_t (usually long, int, long, pointer)
and mpz_t (usually int, int, pointer), this sometimes just produces silently
garbage (e.g. on 64-bit little-endian, where the second int in mpz_t is
the most significant 32-bits of _mpfr_prec and so usually 0 and so _mp_size
is 0 and nothing is dereferenced:
p loop->ext.iterator->step.value.real
$1 = {{_mpfr_prec = 24, _mpfr_sign = 1, _mpfr_exp = 1, _mpfr_d = 0x4eac658}}
p loop->ext.iterator->step.value.integer
$2 = {{_mp_alloc = 24, _mp_size = 0, _mp_d = 0x1}}
or it ICEs somewhere in gmp (on 32-bit little-endian, e.g. i686, or
big-endian, e.g. s390x), because _mp_size is in that case _mpfr_sign but
_mp_d is _mpfr_exp (32-bit little-endian), or _mp_size is least significant
part of _mpfr_prec, in both cases non-zero, but _mp_d is not a usable
pointer in either case.

The following patch fixes that by punting for inner loops with non-integral
iterator.

2026-07-27  Jakub Jelinek  <jakub@redhat.com>

	PR fortran/126303
	* frontend-passes.cc (inner_loop_may_be_skipped): If inner loop iterator
	is not integral, return true.

	* gfortran.dg/pr126303.f: New test.

Reviewed-by: Jerry DeLisle <jvdelisle@gcc.gnu.org>