[llvm] [InstCombine] Prevent infinite loop with two shifts (PR #118806)

[Luke Lau via llvm-commits]

lukel97 wrote:

I think this might have broken some combines with scalable vectors, beforehand this 

```llvm
define <vscale x 4 x i8> @f(<vscale x 4 x i8> %x) {
  %1 = and <vscale x 4 x i8> %x, splat (i8 2)
  %2 = shl <vscale x 4 x i8> %1, splat (i8 2)
  ret <vscale x 4 x i8> %2
}
```

used to get combined to

```llvm
define <vscale x 4 x i8> @f(<vscale x 4 x i8> %x) {
  %1 = shl <vscale x 4 x i8> %x, splat (i8 2)
  %2 = and <vscale x 4 x i8> %1, splat (i8 8)
  ret <vscale x 4 x i8> %2
}
```

But now this fails because a scalable splat vector is a constant expression IIUC. Maybe we need some sort of pattern matcher that matches constant immediates + constant splats?

https://github.com/llvm/llvm-project/pull/118806