[llvm] [DirectX] add GEP i8 legalization (PR #142475)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

the bug I was looking at used allocsa, this one uses a globals its an easy fix to add:
```
if (auto *GV = dyn_cast<GlobalVariable>(BasePtr))
      ElementType = GV->getValueType();
```

However during my testing I found another gap. we still need to figure out how to support constExpr GEPs. 

> > This PR appears to fail to compile this shader: https://godbolt.org/z/9MPc8Wca5
> 
> This shader now compiles, but it currently fails DXIL validation





https://github.com/llvm/llvm-project/pull/142475