[gcc r14-1935] vect: Restore aarch64 bootstrap

[Richard Sandiford]

https://gcc.gnu.org/g:1de281a21e0ad349b5633dc24a4ff23b2c5e04db

commit r14-1935-g1de281a21e0ad349b5633dc24a4ff23b2c5e04db
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Mon Jun 19 09:43:36 2023 +0100

    vect: Restore aarch64 bootstrap
    
    gcc/
            * tree-vect-loop-manip.cc (vect_set_loop_condition_partial_vectors):
            Handle null niters_skip.

Diff:
---
 gcc/tree-vect-loop-manip.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 213d248b485..20f570e4a0d 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -820,7 +820,8 @@ vect_set_loop_condition_partial_vectors (class loop *loop,
   tree ni_actual_type = TREE_TYPE (niters);
   unsigned int ni_actual_precision = TYPE_PRECISION (ni_actual_type);
   tree niters_skip = LOOP_VINFO_MASK_SKIP_NITERS (loop_vinfo);
-  niters_skip = gimple_convert (&preheader_seq, compare_type, niters_skip);
+  if (niters_skip)
+    niters_skip = gimple_convert (&preheader_seq, compare_type, niters_skip);
 
   /* Convert NITERS to the same size as the compare.  */
   if (compare_precision > ni_actual_precision