aarch64: Tweak the cost of elementwise stores

aarch64: Tweak the cost of elementwise stores

When the vectoriser scalarises a strided store, it counts one
scalar_store for each element plus one vec_to_scalar extraction
for each element.  However, extracting element 0 is free on AArch64,
so it should have zero cost.

I don't have a testcase that requires this for existing -mtune
options, but it becomes more important with a later patch.

gcc/
	* config/aarch64/aarch64.c (aarch64_is_store_elt_extraction): New
	function, split out from...
	(aarch64_detect_vector_stmt_subtype): ...here.
	(aarch64_add_stmt_cost): Treat extracting element 0 as free.