poly_int: vect_permute_load/store_chain

poly_int: vect_permute_load/store_chain

The GET_MODE_NUNITS patch made vect_grouped_store_supported and
vect_grouped_load_supported check for a constant number of elements,
so vect_permute_store_chain and vect_permute_load_chain can assert
for that.  This patch adds commentary to that effect; the actual
asserts will be added by a later, more mechanical, patch.

The patch also reorganises the function so that the asserts
are linked specifically to code that builds permute vectors
element-by-element.  This allows a later patch to add support
for some variable-length permutes.

2018-01-03  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_permute_store_chain): Reorganize
	so that both the length == 3 and length != 3 cases set up their
	own permute vectors.  Add comments explaining why we know the
	number of elements is constant.
	(vect_permute_load_chain): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256196