Fortran: improve checking of substring bounds [PR119118]

Fortran: improve checking of substring bounds [PR119118]

After the fix for pr98490 no substring bounds check was generated if the
substring start was not a variable.  While the purpose of that fix was to
suppress a premature check before implied-do indices were substituted, this
prevented a check if the substring start was an expression or a constant.
A better solution is to defer the check until implied-do indices have been
substituted in the start and end expressions.

	PR fortran/119118

gcc/fortran/ChangeLog:

	* dependency.cc (gfc_contains_implied_index_p): Helper function to
	determine if an expression has a dependence on an implied-do index.
	* dependency.h (gfc_contains_implied_index_p): Add prototype.
	* trans-expr.cc (gfc_conv_substring): Adjust logic to not generate
	substring bounds checks before implied-do indices have been
	substituted.

gcc/testsuite/ChangeLog:

	* gfortran.dg/bounds_check_23.f90: Generalize test.
	* gfortran.dg/bounds_check_26.f90: New test.