Don't vectorise single-iteration epilogues

With a later patch I saw a case in which we peeled a single iteration
for gaps but didn't need to peel further iterations to make up a full
vector.  We then tried to vectorise the single-iteration epilogue.

2019-11-06  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vect_analyze_loop): Only try to vectorize
	the epilogue if there are peeled iterations for it to handle.

From-SVN: r277886

gcc/ChangeLog

+2019-11-06  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* tree-vect-loop.c (vect_analyze_loop): Only try to vectorize
+	the epilogue if there are peeled iterations for it to handle.
+
 2019-11-06  Claudiu Zissulescu  <claziss@synopsys.com>
 
 	* config/arc/arc.c (arc_split_ior): Add asserts.

gcc/tree-vect-loop.c

@@ -2459,6 +2459,7 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
 	  vect_epilogues = (!loop->simdlen
 			    && loop->inner == NULL
 			    && PARAM_VALUE (PARAM_VECT_EPILOGUES_NOMASK)
+			    && LOOP_VINFO_PEELING_FOR_NITER (first_loop_vinfo)
 			    /* For now only allow one epilogue loop.  */
 			    && first_loop_vinfo->epilogue_vinfos.is_empty ());