tree-optimization/127588 - avoid UB in epilog IV init computation

The following avoids UB when computing the start value of the epilog
IV value which we compute as init + step * vect-niters where
the step * vect-niters computation can overflow.

	PR tree-optimization/127588
	* tree-vect-loop-manip.cc (vect_update_ivs_after_vectorizer):
	Use an unsigned type to compute the epilog start value.

	* gcc.dg/vect/vect-iv-15.c: New testcase.