tree-optimization/109747 - SLP cost of CTORs

tree-optimization/109747 - SLP cost of CTORs

The x86 backend looks at the SLP node passed to the add_stmt_cost
hook when costing vec_construct, looking for elements that require
a move from a GPR to a vector register and cost that.  But since
vect_prologue_cost_for_slp decomposes the cost for an external
SLP node into individual pieces this cost gets applied N times
without a chance for the backend to know it's just dealing with
a part of the SLP node.  Just looking at a part is also not perfect
since the GPR to XMM move cost applies only once per distinct
element so handling the whole SLP node one more correctly reflects
cost (albeit without considering other external SLP nodes).

The following addresses the issue by passing down the SLP node
only for one piece and nullptr for the rest.  The x86 backend
is currently the only one looking at it.

In the future the cost of external elements is something to deal
with globally but that would require the full SLP tree be available
to costing.

It's difficult to write a testcase, at the tipping point not
vectorizing is better so I'll followup with x86 specific adjustments
and will see to add a testcase later.

	PR tree-optimization/109747
	* tree-vect-slp.cc (vect_prologue_cost_for_slp): Pass down
	the SLP node only once to the cost hook.