[llvm] [GISel] Fold bitreverse(shl/srl(bitreverse(x),y)) -> srl/shl(x,y) (PR #91355)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

https://llvm.org/docs/GlobalISel/MIRPatterns.html#gallery

```
(match (G_SHL $src, $x, $y), (G_BITREVERSE $root, $src)), [{ Helper.match .. }],
```

https://github.com/llvm/llvm-project/pull/91355