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

[Helena Kotas via llvm-commits]

hekota wrote:

> might be good to have a reverversebits error case so we know dxil doesn't get generated for floats.

I assumed we are only adding lowering error cases when the llvm intrinsic supports more types than DXIL. For example in the `cos_error.ll` there is error case for the `double` overload, but not for any integer overloads because `llvm.cos` does not support integers. The `llvm.bitreverse` intrinsics only has only integer overloads and `opt` fails to even load the code with `llvm.bitreverse.f32`, let alone reach the `dxil-op-lower` pass.

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