[llvm] [CGP] Eliminate noop bitcasts (PR #146961)

[Philip Reames via llvm-commits]

preames wrote:

> Aren't no-op bitcasts the way how ConstantHoisting conveys opaque values to the backend?

I weakly believe that the opaque constant case is covered by the check just above:
```
if (isa<Constant>(CI->getOperand(0)))
      return AnyChange;
```

I say weakly here because I'm not familiar with constant hoisting beyond a cursory level, and it might be I'm wrong here.  I did a quick search through the code, but if there's something more subtle going on - maybe around the constant expr stuff? - I might have missed it.  

https://github.com/llvm/llvm-project/pull/146961