[llvm] [InstCombine] Propagate exact flags in shift-combine transforms (PR #88340)

[Nikita Popov via llvm-commits]

nikic wrote:

The test failures in CI are still there:

```
Failed Tests (2):
  LLVM :: Transforms/InstCombine/lshr.ll
  LLVM :: Transforms/InstCombine/shift.ll
```

It would be good to split this PR into separate PRs that handle one fold each. Otherwise it is hard to check whether all cases are covered. Because the tests are not pre-committed, it's also hard to see which of the changes are necessary. E.g. I think the explicit trunc flag preservation may not be needed, as it will get inferred anyway? See https://llvm.godbolt.org/z/MWefaxabj.

https://github.com/llvm/llvm-project/pull/88340