[Vectorizer] Use a VEC_PERM_EXPR instead of VEC_RSHIFT_EXPR; expand appropriate VEC_PERM_EXPRs using vec_shr_optab

	* optabs.c (can_vec_perm_p): Update comment, does not consider vec_shr.
	(shift_amt_for_vec_perm_mask): New.
	(expand_vec_perm_1): Use vec_shr_optab if second vector is const0_rtx
	and mask appropriate.

	* tree-vect-loop.c (calc_vec_perm_mask_for_shift): New.
	(have_whole_vector_shift): New.
	(vect_model_reduction_cost): Call have_whole_vector_shift instead of
	looking for vec_shr_optab.
	(vect_create_epilog_for_reduction): Likewise; also rename local variable
	have_whole_vector_shift to reduce_with_shift; output VEC_PERM_EXPRs
	instead of VEC_RSHIFT_EXPRs.

	* tree-vect-stmts.c (vect_gen_perm_mask_checked): Extend comment.

From-SVN: r217509