[llvm] [DirectX] Support ConstExpr GEPs (PR #148986)

[Farzon Lotfi via llvm-commits]

farzonl wrote:


This change does fix a bug, but I think we need to file a different ticket for this case:
```hlsl
groupshared uint4 aTile[10][10];

[numthreads(1, 1, 1)]
void CSMain() {
    uint4 aFragPacked = aTile[1][2];
}
```
Need to rename the branch and then we don't have to call this a partial fix.

https://github.com/llvm/llvm-project/pull/148986