Fix gather/scatter check when updating a vector epilogue loop

Fix gather/scatter check when updating a vector epilogue loop

update_epilogue_loop_vinfo applies SSA renmaing to the DR_REF of a
gather or scatter, so that vect_check_gather_scatter continues to work.
However, we sometimes also rely on vect_check_gather_scatter when
using gathers and scatters to implement strided accesses.

This showed up on existing tests when testing with fixed-length
-msve-vector-bits=128.

2020-01-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (update_epilogue_loop_vinfo): Update DR_REF
	for any type of gather or scatter, including strided accesses.

From-SVN: r280111