[llvm] [PowerPC] Replace vspltisw+vadduwm instructions with xxleqv+vsubuwm for adding the vector {1, 1, 1, 1} (PR #160882)

[via llvm-commits]

Himadhith wrote:

This patch does not handle `v1i128` vector type because it does not emit the instruction `vspltisw`. 

```
# %bb.0:                                # %entry
        addis 3, 2, .LCPI4_0 at toc@ha
        addi 3, 3, .LCPI4_0 at toc@l
        lxvd2x 0, 0, 3
        xxswapd 35, 0
        vadduqm 2, 2, 3
        blr
        .long   0
        .quad   0
```

https://github.com/llvm/llvm-project/pull/160882