re PR c++/87481 (Endless loop with optimisation in C++17)

re PR c++/87481 (Endless loop with optimisation in C++17)

	PR c++/87481
	* doc/invoke.texi (-fconstexpr-ops-limit=): Document.

	* c.opt (-fconstexpr-ops-limit=): New option.

	* constexpr.c (struct constexpr_ctx): Add constexpr_ops_count member.
	(cxx_eval_constant_expression): When not skipping, not constant class
	or location wrapper, increment *ctx->constexpr_ops_count and if it is
	above constexpr_loop_nest_limit, diagnose failure.
	(cxx_eval_outermost_constant_expr): Add constexpr_ops_count and
	initialize ctx.constexpr_ops_count to its address.
	(is_sub_constant_expr): Likewise.

	* g++.dg/cpp1y/constexpr-87481.C: New test.

From-SVN: r269874