Fortran: fix ICE in verify_gimple_in_seq with substrings [PR120743]

Fortran: fix ICE in verify_gimple_in_seq with substrings [PR120743]

	PR fortran/120743

gcc/fortran/ChangeLog:

	* trans-expr.cc (gfc_conv_substring): Substring indices are of
	type gfc_charlen_type_node.  Convert to size_type_node for
	pointer arithmetic only after offset adjustments have been made.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr120743.f90: New test.

Co-authored-by: Jerry DeLisle <jvdelisle@gcc.gnu.org>
Co-authored-by: Mikael Morin <mikael@gcc.gnu.org>