Add copy_rtx call to RTL loop unroller

Add copy_rtx call to RTL loop unroller

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