tree-optimization/117875 - missed SLP vectorization

There's a discrepancy in SLP vs non-SLP vectorization that SLP build
does not handle plain SSA copies (which should have been elimiated
earlier).  But this now bites back since non-SLP happily handles them,
causing a regression with --param vect-force-slp=1 which is now default,
resulting in a big performance regression in 456.hmmer.

So the following restores parity between SLP and non-SLP here, defering
the missed copy elimination to later (PR118565).

	PR tree-optimization/117875
	* tree-vect-slp.cc (vect_build_slp_tree_1): Handle SSA copies.