tree-optimization/97732 - fix init of SLP induction vectorization

tree-optimization/97732 - fix init of SLP induction vectorization

This PR exposes two issues - one that the vector builder treats
&x as eligible for VECTOR_CST elements and one that SLP induction
vectorization forgets to convert init elements to the vector
component type which makes a difference for pointer vs. integer.

2020-11-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97732
	* tree-vect-loop.c (vectorizable_induction): Convert the
	init elements to the vector component type.
	* gimple-fold.c (gimple_build_vector): Use CONSTANT_CLASS_P
	rather than TREE_CONSTANT to determine if elements are
	eligible for VECTOR_CSTs.

	* gcc.dg/vect/bb-slp-pr97732.c: New testcase.