Do less redundant vect_transform_slp_perm_load calls

Do less redundant vect_transform_slp_perm_load calls

The following tries to do vect_transform_slp_perm_load exactly
once during analysis and once during transform.  There's a 2nd
case left during analysis in get_load_store_type.  Temporarily
this records n_perms in the load-store info and verifies that
against the value computed at transform stage.

	* tree-vectorizer.h (vect_load_store_data::n_perms): New.
	* tree-vect-stmts.cc (vectorizable_load): Analyze
	SLP_TREE_LOAD_PERMUTATION only once and remember n_perms.
	Verify the transform-time n_perms against the value stored
	during analysis.