tree-optimization/127195 - ICE with vect_create_constant_vectors

With excess lanes present we miscount the number of copies of the
SLP elements which eventually results in ctor_seq stmts not being
emitted.

	PR tree-optimization/127195
	* tree-vect-slp.cc (vect_create_constant_vectors):
	Properly compute number_of_copies, assert we end up
	on a vector boundary.

	* gcc.dg/vect/vect-pr127195.c: New testcase.