Allow single-element interleaving for non-power-of-2 strides

Allow single-element interleaving for non-power-of-2 strides

This allows LD3 to be used for isolated a[i * 3] accesses, in a similar
way to the current a[i * 2] and a[i * 4] for LD2 and LD4 respectively.
Given the problems with the cost model underestimating the cost of
elementwise accesses, the patch continues to reject the VMAT_ELEMENTWISE
cases that are currently rejected.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-data-refs.c (vect_analyze_group_access_1): Allow
	single-element interleaving even if the size is not a power of 2.
	* tree-vect-stmts.c (get_load_store_type): Disallow elementwise
	accesses for single-element interleaving if the group size is
	not a power of 2.

gcc/testsuite/
	* gcc.target/aarch64/sve/struct_vect_18.c: New test.
	* gcc.target/aarch64/sve/struct_vect_18_run.c: Likewise.
	* gcc.target/aarch64/sve/struct_vect_19.c: Likewise.
	* gcc.target/aarch64/sve/struct_vect_19_run.c: Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256634 138bc75d-0d04-0410-961f-82ee72b054a4