[llvm] [DirectX] legalize memset (PR #136244)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

The   HLSL Test Github action failures are because of:
```
Clang-Unit :: ./AllClangUnitTests/TimeProfilerTest/ConstantEvaluationC99
Clang-Unit :: ./AllClangUnitTests/TimeProfilerTest/ConstantEvaluationCxx20
```
This change doesn't touch that so  will likely get fixed the next time i rebase.

https://github.com/llvm/llvm-project/pull/136244