[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