[llvm] [InstCombine] Fold (1 << a) to (a + 1) and (2 >> a) to (2 - a) when a is in [0,1] (PR #207108)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw requested changes to this pull request.

A correctness issue was identified by [llvm-hackme](https://github.com/dtcxzyw/llvm-hackme). Please see the [issue comment](https://github.com/llvm/llvm-project/pull/207108#issuecomment-4861529745) for details.

https://github.com/llvm/llvm-project/pull/207108