vect: Add costing_for_scalar parameter to init_cost hook

rs6000 port function rs6000_density_test wants to differentiate the
current cost model is for the scalar version of a loop or block, or
the vector version.  As Richi suggested, this patch introduces one
new parameter costing_for_scalar to init_cost hook to pass down this
information explicitly.

gcc/ChangeLog:

	* doc/tm.texi: Regenerated.
	* target.def (init_cost): Add new parameter costing_for_scalar.
	* targhooks.c (default_init_cost): Adjust for new parameter.
	* targhooks.h (default_init_cost): Likewise.
	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Likewise.
	(vect_compute_single_scalar_iteration_cost): Likewise.
	(vect_analyze_loop_2): Likewise.
	* tree-vect-slp.c (_bb_vec_info::_bb_vec_info): Likewise.
	(vect_bb_vectorization_profitable_p): Likewise.
	* tree-vectorizer.h (init_cost): Likewise.
	* config/aarch64/aarch64.c (aarch64_init_cost): Likewise.
	* config/i386/i386.c (ix86_init_cost): Likewise.
	* config/rs6000/rs6000.c (rs6000_init_cost): Likewise.