cp-gimplify.c (cp_maybe_instrument_return): Don't add __builtin_unreachable if -O0 or if -fsanitize=unreachable.

	* cp-gimplify.c (cp_maybe_instrument_return): Don't add
	__builtin_unreachable if -O0 or if -fsanitize=unreachable.

	* g++.dg/missing-return.C: Add -O to dg-options.

From-SVN: r255403