vect: Support gather loads with SLP

This patch adds SLP support for IFN_GATHER_LOAD.  Like the SLP
support for IFN_MASK_LOAD, it works by treating only some of the
arguments as child nodes.  Unlike IFN_MASK_LOAD, it requires the
other arguments (base, scale, and extension type) to be the same
for all calls in the group.  It does not require/expect the loads
to be in a group (which probably wouldn't make sense for gathers).

I was worried about the possible alias effect of moving gathers
around to be part of the same SLP group.  The patch therefore
makes vect_analyze_data_ref_dependence treat gathers and scatters
as a top-level concern, punting if the accesses aren't completely
independent and if the user hasn't told us that a particular
VF is safe.  I think in practice we already punted in the same
circumstances; the idea is just to make it more explicit.

gcc/
	PR tree-optimization/102467
	* doc/sourcebuild.texi (vect_gather_load_ifn): Document.
	* tree-vect-data-refs.c (vect_analyze_data_ref_dependence):
	Commonize safelen handling.  Punt for anything involving
	gathers and scatters unless safelen says otherwise.
	* tree-vect-slp.c (arg1_map): New variable.
	(vect_get_operand_map): Handle IFN_GATHER_LOAD.
	(vect_build_slp_tree_1): Likewise.
	(vect_build_slp_tree_2): Likewise.
	(compatible_calls_p): If vect_get_operand_map returns nonnull,
	check that any skipped arguments are equal.
	(vect_slp_analyze_node_operations_1): Tighten reduction check.
	* tree-vect-stmts.c (check_load_store_for_partial_vectors): Take
	an ncopies argument.
	(vect_get_gather_scatter_ops): Take slp_node and ncopies arguments.
	Handle SLP nodes.
	(vectorizable_store, vectorizable_load): Adjust accordingly.

gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_vect_gather_load_ifn): New target test.
	* gcc.dg/vect/vect-gather-1.c: New test.
	* gcc.dg/vect/vect-gather-2.c: Likewise.
	* gcc.target/aarch64/sve/gather_load_11.c: Likewise.

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