[llvm] [AArch64][GlobalISel] Add legalisation to G_EXRACT_SUBVECTOR (PR #207956)

[David Green via llvm-commits]

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davemgreen wrote:

As we are only clamping high at the moment, this can be
```
      .clampMaxNumElements(0, s8, 16)
      .clampMaxNumElements(0, s16, 8)
      .clampMaxNumElements(0, s32, 4)
```

https://github.com/llvm/llvm-project/pull/207956