This is needed if the step is an unshared constant, like many
(const ...)s are.  Without this patch, libgfortran would fail
to build for SVE.

2017-09-21  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* loop-unroll.c (split_iv): Call copy_rtx on the step.

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

From-SVN: r253058