[llvm] [DXIL] Add lowering for `reversebits` and `trunc` (PR #86909)

[Damyan Pepper via llvm-commits]

damyanp wrote:

I think you need to update the PR description so it says `Fixes #86582` or `Closes #86582`.  `Completes #86582` doesn't seem to get picked up. https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/using-keywords-in-issues-and-pull-requests

https://github.com/llvm/llvm-project/pull/86909