wi::add_large and wi::sub_large weren't setting the overflow bit
correctly for unsigned operations if the result needed fewer HWIs
than the precision.

2017-11-22  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR middle-end/82547
	* wide-int.cc (wi::add_large, wi::sub_large): Fix overflow detection
	for unsigned values with fewer HWIs than the precision.
	(test_overflow): New function.
	(wide_int_cc_tests): Call it.

From-SVN: r255059