Improve ivopts handling of forced scales

Improve ivopts handling of forced scales

This patch improves the ivopts address cost calculation for modes
in which an index must be scaled rather than unscaled.  Previously
we would only try the scaled form if the unscaled form was valid.

Many of the SVE tests rely on this when matching scaled indices.

2017-11-09  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-ssa-loop-ivopts.c (get_address_cost): Try using a
	scaled index even if the unscaled address was invalid.
	Don't increase the complexity of using a scale in that case.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r254585