So far we've used integer modes for LD[234] and ST[234] arrays.
That doesn't scale well to SVE, since the sizes aren't fixed at
compile time (and even if they were, we wouldn't want integers
to be so wide).

This patch lets the target use double-, triple- and quadruple-length
vectors instead.

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

gcc/
	* target.def (array_mode): New target hook.
	* doc/tm.texi.in (TARGET_ARRAY_MODE): New hook.
	* doc/tm.texi: Regenerate.
	* hooks.h (hook_optmode_mode_uhwi_none): Declare.
	* hooks.c (hook_optmode_mode_uhwi_none): New function.
	* tree-vect-data-refs.c (vect_lanes_optab_supported_p): Use
	targetm.array_mode.
	* stor-layout.c (mode_for_array): Likewise.  Support polynomial
	type sizes.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256617