Sorry for the delay, just been reminded I still had this patch 
outstanding from last stage 1. Hopefully since it has been mostly 
reviewed it could go in for this stage 1?

I addressed the comments and gave the slp-part of vectorizable_call some 
TLC to make it work.

I also changed vect_get_slp_defs as I noticed that the call from 
vectorizable_call was creating an auto_vec with 'nargs' that might be 
less than the number of children in the slp_node, so that quick_push 
might not be safe as is, so I added the reserve (n) to ensure it's safe 
to push. I didn't actually come across any failure because of it though. 
Happy to split this into a separate patch if needed.

Bootstrapped and regression tested on aarch64-none-linux-gnu and 
x86_64-pc-linux-gnu.

OK for trunk?

gcc/ChangeLog:

         * config/aarch64/aarch64.md (ftrunc<mode><frintnz_mode>2): New 
pattern.
         * config/aarch64/iterators.md (FRINTNZ): New iterator.
         (frintnz_mode): New int attribute.
         (VSFDF): Make iterator conditional.
         * internal-fn.def (FTRUNC_INT): New IFN.
         * internal-fn.cc (ftrunc_int_direct): New define.
         (expand_ftrunc_int_optab_fn): New custom expander.
         (direct_ftrunc_int_optab_supported_p): New supported_p.
         * internal-fn.h (direct_internal_fn_info): Add new member
         type1_is_scalar_p.
         * match.pd: Add to the existing TRUNC pattern match.
         * optabs.def (ftrunc_int): New entry.
         * stor-layout.h (element_precision): Moved from here...
         * tree.h (element_precision): ... to here.
         (element_type): New declaration.
         * tree.cc (element_type): New function.
         (element_precision): Changed to use element_type.
         * tree-vect-stmts.cc (vectorizable_internal_function): Add 
support for
         IFNs with different input types.
         (vect_get_scalar_oprnds): New function.
         (vectorizable_call): Teach to handle IFN_FTRUNC_INT.
         * tree-vect-slp.cc (check_scalar_arg_ok): New function.
         (vect_slp_analyze_node_operations): Use check_scalar_arg_ok.
         (vect_get_slp_defs): Ensure vec_oprnds has enough slots to push.
         * doc/md.texi: New entry for ftrunc pattern name.
         * doc/sourcebuild.texi (aarch64_frintzx_ok): New target.

gcc/testsuite/ChangeLog:

         * gcc.target/aarch64/merge_trunc1.c: Adapted to skip if frintnz 
instructions available.
         * lib/target-supports.exp: Added aarch64_frintnzx_ok target and 
aarch64_frintz options.
         * gcc.target/aarch64/frintnz.c: New test.
         * gcc.target/aarch64/frintnz_vec.c: New test.
         * gcc.target/aarch64/frintnz_slp.c: New test.