Improve DCE of dead parts of a permute chain

Improve DCE of dead parts of a permute chain

gcc.dg/vect/bb-slp-pr78205.c is reported to have regressed with
the PR113073 change and in the end it's due to the DCE performed
by vect_transform_slp_perm_load_1 being imperfect.  The following
enhances it to also cover the CTOR and VIEW_CONVERT operations that
might be involved.

	* tree-vect-slp.cc (vect_transform_slp_perm_load_1): Also handle
	CTOR and VIEW_CONVERT up to the load when performing chain DCE.