vect: Add target hook to prefer gather/scatter instructions

vect: Add target hook to prefer gather/scatter instructions

For AMD GCN, the instructions available for loading/storing vectors are
always scatter/gather operations (i.e. there are separate addresses for
each vector lane), so the current heuristic to avoid gather/scatter
operations with too many elements in get_group_load_store_type is
counterproductive. Avoiding such operations in that function can
subsequently lead to a missed vectorization opportunity whereby later
analyses in the vectorizer try to use a very wide array type which is
not available on this target, and thus it bails out.

This patch adds a target hook to override the "single_element_p"
heuristic in the function as a target hook, and activates it for GCN. This
allows much better code to be generated for affected loops.

Co-authored-by:  Julian Brown  <julian@codesourcery.com>
gcc/
	* doc/tm.texi.in (TARGET_VECTORIZE_PREFER_GATHER_SCATTER): Add
	documentation hook.
	* doc/tm.texi: Regenerate.
	* target.def (prefer_gather_scatter): Add target hook under vectorizer.
	* hooks.cc (hook_bool_mode_int_unsigned_false): New function.
	* hooks.h (hook_bool_mode_int_unsigned_false): New prototype.
	* tree-vect-stmts.cc (vect_use_strided_gather_scatters_p): Add
	parameters group_size and single_element_p, and rework to use
	targetm.vectorize.prefer_gather_scatter.
	(get_group_load_store_type): Move some of the condition into
	vect_use_strided_gather_scatters_p.
	* config/gcn/gcn.cc (gcn_prefer_gather_scatter): New function.
	(TARGET_VECTORIZE_PREFER_GATHER_SCATTER): Define hook.

(cherry picked from commit 36c5a7aa9a6dbaed07e3a2482c66743ddcb3e776)