[llvm] [LoongArch] Optimize extractelement containing variable index (PR #151475)

[via llvm-commits]

heiher wrote:

Are these okay?

```
xr0: v32i8
a0 : index
                                   L/T
xvpermi.q       xr1, xr0, 1        3/4
xvreplgr2vr.b   xr2, a0            3/1
xvshuf.b        xr2, xr1, xr0, xr2 1/2
Total:                             7/1

xr0: v16i16
a0 : index
                                   L/T
xvpermi.q       xr1, xr0, 1        3/4
xvreplgr2vr.h   xr2, a0            3/1
xvshuf.h        xr2, xr1, xr0      1/2
Total:                             7/1

xr0: v8i32
a0 : index
                                   L/T
xvpermi.q       xr1, xr0, 1        3/4
xvreplgr2vr.w   xr2, a0            2/1
xvshuf.w        xr2, xr1, xr0      1/2
Total:                             6/1

xr0: v4i64
a0 : index
                                   L/T
xvpermi.q       xr1, xr0, 1        3/4
xvreplgr2vr.d   xr2, a0            2/1
xvshuf.d        xr2, xr1, xr0      1/2
Total:                             6/1
```

https://github.com/llvm/llvm-project/pull/151475