[2/7] Clean up vectorizer load/store costs

[2/7] Clean up vectorizer load/store costs

Add a bit more commentary and try to make the structure more obvious.
The horrendous:

      if (grouped_access_p
          && represents_group_p
          && !store_lanes_p
          && !STMT_VINFO_STRIDED_P (stmt_info)
          && !slp_node)

checks go away in patch 6.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/
	* tree-vect-stmts.c (vect_cost_group_size): Delete.
	(vect_model_store_cost): Avoid calling it.  Use first_stmt_p
	variable to indicate when once-per-group costs are being used.
	(vect_model_load_cost): Likewise.  Fix comment and misindented code.

From-SVN: r238034