tree-optimization/99947 - avoid v.safe_push (v[0])

This avoids (again) the C++ pitfall of pushing a reference to
sth being reallocated.

2021-04-07  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/99947
	* tree-vect-loop.c (vectorizable_induction): Pre-allocate
	steps vector to avoid pushing elements from the reallocated
	vector.

	* gcc.dg/torture/pr99947.c: New testcase.