Unshare DR_STEP before gimplifying it

In this testcase we use an unmasked SVE loop with an Advanced SIMD
epilogue (because we don't yet support fully-masked downward loops).
The main loop uses a gather load for the strided access while the
epilogue loop builds the access from scalars instead.  In both cases
we gimplify expressions based on the DR_STEP and insert them in the
loop preheader.

The problem was that the gather load code didn't copy the DR_STEP before
gimplifying it, meaning that the epilogue loop tried to reuse a result
from the (non-dominating) main loop preheader.

It looks at first glance like there could be other instances of this too,
but this patch just deals with the gather/scatter case.

2019-12-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vect_get_strided_load_store_ops): Copy
	DR_STEP before gimplifying it.

gcc/testsuite/
	* gcc.dg/vect/vect-strided-epilogue-1.c: New test.

From-SVN: r279753