Fortran: Minor PDT cleanup and fix in gfc_simplify_exp [PR115315]

2026-03-26  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
	PR fortran/115315
	* decl.cc (insert_parameter_exprs): Make strcmp condition more
	concise.
	(gfc_get_pdt_instance): Use gf_replace_expr where possible and
	use return value of gfc_simplify_expr. Correct error in which
	params->expr was being simplified instead of c2->initializer.
	* expr.cc (gfc_simplify_expr): If the substring 'start' value
	is less than zero, it is clearly out of range and so return
	false.

gcc/testsuite/
	PR fortran/115315
	* gfortran.dg/pdt_90.f03: New test.