In the testcase we were trying to group two gather loads, even though
that isn't supported.  Fixed by explicitly disallowing grouping of
gathers and scatters.

This problem didn't show up on SVE because there we convert to
IFN_GATHER_LOAD/IFN_SCATTER_STORE pattern statements, which fail
the can_group_stmts_p check.

2018-01-16  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vect-data-refs.c (vect_analyze_data_ref_accesses):

gcc/testsuite/
	* gcc.dg/torture/pr83847.c: New test.

From-SVN: r256730