ivopts: replace constant_multiple_of with aff_combination_constant_multiple_p [PR114932]

The current implementation of constant_multiple_of is doing a more limited
version of aff_combination_constant_multiple_p.

The only non-debug usage of constant_multiple_of will proceed with the values
as affine trees.  There is scope for further optimization here, namely I believe
that if constant_multiple_of returns the aff_tree after the conversion then
get_computation_aff_1 can use it instead of manually creating the aff_tree.

However I think it makes sense to first commit this smaller change and then
incrementally change things.

gcc/ChangeLog:

	PR tree-optimization/114932
	* tree-ssa-loop-ivopts.cc (constant_multiple_of): Use
	aff_combination_constant_multiple_p instead.