cvt.c (convert_to_void): Only warn about COND_EXPR if neither the second nor third operand has side effects.

cp/:
	* cvt.c (convert_to_void): Only warn about COND_EXPR if neither
	the second nor third operand has side effects.
testsuite/:
	* g++.dg/warn/Wunused-16.C: New testcase.

From-SVN: r148950