[Bug target/116507] New: [15 Regression] movhi_aarch64 should use fmov if FP16

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116507

            Bug ID: 116507
           Summary: [15 Regression] movhi_aarch64 should use fmov if FP16
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: pinskia at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64

Take:
```
_Float16 fs(unsigned short a)
{
  return __builtin_bit_cast(_Float16, a);
}
```

GCC 14 produced:
```
        fmov    h0, w0
        ret
```

Which is decent.

While GCC 15 produces:
```
        dup     v0.4h, w0
        ret
```

This is due to different mode being used in GCC 14 vs 15 for the allocated
register.
In GCC 14, HF was used while on the trunk, HI is used.

Options used: `-O2 -march=armv9-a+fp16` .

Note clang produces always:
```
        fmov    s0, w0
        ret
```

But I am not sure if that is correct though.

I will be fixing this one ...