[llvm] [DXIL] Implement pow lowering (PR #86733)

[Paul Kirth via llvm-commits]

ilovepi wrote:

Hi, I'm seeing an assertion failure that seems to be due to this patch. 
I bisected it to this patch. I'm including a reproducer (not reduced) and my bisect script.

Failing bot: https://ci.chromium.org/ui/p/fuchsia/builders/ci/clang_toolchain.ci.core.arm64-debug-subbuild/b8749763985778255185/overview
Logs: https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8749760927129551041/+/u/clang/build/stdout


I bisected with
```
git bisect good  0f61051f541a5b8cfce25c84262dfdbadb9ca688
git bisect bad d3f6a88a1fb36f94c71940514e576821c6cc3ade
git bisect run bisect.sh
```
I checked the good and bad SHA's multiple times, and the good commit is one commit prior to yours.

Note if you want to use the bisect script you may need to update paths. The `.cpp` and other `.sh` are from the generated reproducer, so should work if you update the path to clang in the script.

I have creduce running now, so I can probably post a smaller reproducer in a while.

In the meantime, can you take a look or possibly revert this until a fix is ready?

[repro.zip](https://github.com/llvm/llvm-project/files/15100080/repro.zip)


https://github.com/llvm/llvm-project/pull/86733