vect: Refactor code for index == count in vect_transform_slp_perm_load_1

vect: Refactor code for index == count in vect_transform_slp_perm_load_1

This patch is to refactor the handlings for the case (index
== count) in a loop of vect_transform_slp_perm_load_1, in
order to prepare a subsequent adjustment on *nperm.  This
patch doesn't have any functional changes.

Basically this is to rewrite two if below:

  if (index == count && !noop_p)
    {
       // A ...
       // ++*n_perms;
    }

  if (index == count)
    {
       if (!analyze_only)
	 {
	    if (!noop_p)
	       // B1 ...

	    // B2 ...

	    for ...
	      {
		 if (!noop_p)
		    // B3 building VEC_PERM_EXPR
		 else
		    // B4 building nothing (no uses for B2 and its seq)
	      }
	 }
       // B5
    }

into one hunk below:

  if (index == count)
    {
       if (!noop_p)
	 {
	   // A ...
	   // ++*n_perms;

	   if (!analyze_only)
	     {
		// B1 ...
		// B2 ...
		for ...
		   // B3 building VEC_PERM_EXPR
	     }
	 }
       else if (!analyze_only)
	 {
	    // no B2 since no any further uses here.
	    for ...
	      // B4 building nothing
	 }
	// B5 ...
    }

gcc/ChangeLog:

	* tree-vect-slp.cc (vect_transform_slp_perm_load_1): Refactor the
	handling for the case index == count.