Add missing alignment checks in epilogue loop vectorisation (PR 86877)

Add missing alignment checks in epilogue loop vectorisation (PR 86877)

Epilogue loop vectorisation skips vect_enhance_data_refs_alignment
since it doesn't make sense to version or peel the epilogue loop
(that will already have happened for the main loop).  But this means
that it also fails to check whether the accesses are suitably aligned
for the new vector subarch.

We don't seem to carry alignment information from the (potentially
peeled or versioned) main loop to the epilogue loop, which would be
good to fix at some point.  I think we want this patch regardless,
since there's no guarantee that the alignment requirements are the
same for every subarch.

2018-09-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86877
	* tree-vect-loop.c (vect_analyze_loop_2): Call
	vect_verify_datarefs_alignment.

gcc/testsuite/
	PR tree-optimization/86877
	* gfortran.dg/vect/vect-8-epilogue.F90: New test.

(cherry picked from commit 508a909eca536f7f6a60af9bd7ecea761bd2e8f1)