[llvm] [GlobalISel][AArch64] Legalize G_SPLAT_VECTOR (PR #114006)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

This pattern looks fine to select `{nxv8s16, s16} `, but it fails:
```
 def : Pat<(nxv8f16 (splat_vector (f16 FPR16:$src))),
            (DUP_ZZI_H (INSERT_SUBREG (IMPLICIT_DEF), FPR16:$src, hsub), 0)>;
```


https://github.com/llvm/llvm-project/pull/114006