Make sure we mark the hw-loop labels as beeing used.

gcc/
2017-09-19  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.c (hwloop_optimize): Update hw-loop's end/start
	labels number of usages.

gcc/testsuite
2017-09-19  Claudiu Zissulescu  <claziss@synopsys.com>

	* gcc.target/arc/loop-2.cpp: New test.

From-SVN: r255274