Fix SLP downward group access classification [PR92420]

Fix SLP downward group access classification [PR92420]

This PR was caused by the SLP handling in get_group_load_store_type
returning VMAT_CONTIGUOUS rather than VMAT_CONTIGUOUS_REVERSE for
downward groups.

A more elaborate fix would be to try to combine the reverse permutation
into SLP_TREE_LOAD_PERMUTATION for loads, but that's really a follow-on
optimisation and not backport material.  It might also not necessarily
be a win, if the target supports (say) reversing and odd/even swaps
as independent permutes but doesn't recognise the combined form.

2020-02-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	Backport from mainline
	2019-11-11  Richard Sandiford  <richard.sandiford@arm.com>

	PR tree-optimization/92420
	* tree-vect-stmts.c (get_negative_load_store_type): Move further
	up file.
	(get_group_load_store_type): Use it for reversed SLP accesses.

gcc/testsuite/
	PR tree-optimization/92420
	* gcc.dg/vect/pr92420.c: New test.