[Bug target/113870] New: Add V2HF support

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113870

            Bug ID: 113870
           Summary: Add V2HF support
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64

Take:
```
void f( _Float16 * __restrict  a, float * __restrict b)
{

 b[0] = a[0];
 b[1] = a[1];
}

```
We should produce:
```
        ldr     s0, [x0]
        fcvtl   v0.4s, v0.4h
        str     d0, [x1]
        ret
```

But this requires V2HF support. I am not 100% sure if this is worth adding.
Adding V2HI and V4QI is worth it though (I have a patch for that).

There are not enough benchmarks that use _Float16 to say if it is worth it or
not.