[llvm] [AArch64] Improve lowering of truncating build vectors (PR #81960)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Can you make sure there are tests for things like this too, along with other type maybe using insertelement/extractelement?
```
define <4 x i16> @test(ptr %p, ptr %q) {
  %a = load <2 x i16>, ptr %p
  %b = load <2 x i16>, ptr %q
  %c = shufflevector <2 x i16> %a, <2 x i16> %b, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
  ret <4 x i16> %c
}
```

https://github.com/llvm/llvm-project/pull/81960