[llvm] [DirectX] Simplify and correct the flattening of GEPs in DXILFlattenArrays (PR #146173)

[Deric C. via llvm-commits]

Icohedron wrote:

I'm not sure why the HLSL Tests GitHub workflow is failing on `LLVM-Unit :: Analysis/./AnalysisTests/IR2VecTest/CreateInvalidMode`. I am not able to reproduce the failure on my end.
```
> build/unittests/Analysis/./AnalysisTests --gtest_filter=IR2VecTest.CreateInvalidMode
Note: Google Test filter = IR2VecTest.CreateInvalidMode
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from IR2VecTest
[ RUN      ] IR2VecTest.CreateInvalidMode
[       OK ] IR2VecTest.CreateInvalidMode (0 ms)
[----------] 1 test from IR2VecTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 1 test.
```

https://github.com/llvm/llvm-project/pull/146173