vect: Account for unused IFN_LOAD_LANES results

vect: Account for unused IFN_LOAD_LANES results

At the moment, if we use only one vector of an LD4 result,
we'll treat the LD4 as having the cost of a single load.
But all 4 loads and any associated permutes take place
regardless of which results are actually used.

This patch therefore counts the cost of unused LOAD_LANES
results against the first statement in a group.  An alternative
would be to multiply the ncopies of the first stmt by the group
size and treat other stmts in the group as having zero cost,
but I thought that might be more surprising when reading dumps.

gcc/
	* tree-vect-stmts.c (vect_model_load_cost): Account for unused
	IFN_LOAD_LANES results.

gcc/testsuite/
	* gcc.target/aarch64/sve/cost_model_11.c: New test.
	* gcc.target/aarch64/sve/mask_struct_load_5.c: Use
	-fno-vect-cost-model.