[Bug target/113857] fmov should be used to zero the upper bits of the vector register

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113857

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Without f16 extension, s64 could be done as:
```
and x0, x0, 0xffff
fmov s0, x0
```

Similarly if we are doing a `c64` .