build_range_check explicitly allows LOW and HIGH to be a different type
from EXP, so we need to use w::to_widest when comparing a value based on
HIGH with a value based on EXP's type.

2017-10-04  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/82413
	* fold-const.c (build_range_check): Use widest_int when comparing
	the maximum ETYPE value with HIGH.

gcc/testsuite/
	PR tree-optimization/82413
	* g++.dg/pr82413.C: New test.

From-SVN: r253401