[DirectX] Fix crash in DXILFlattenArrays for function declarations (#116690)

We were skipping intrinsics here, but really we need to skip all
function declarations - if the function doesn't have a

[DirectX] Fix crash in DXILFlattenArrays for function declarations (#116690)

We were skipping intrinsics here, but really we need to skip all
function declarations - if the function doesn't have a body there's
nothing to walk.

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[DirectX] Flatten arrays (#114332)

- Relevant piece is `DXILFlattenArrays.cpp`
- Loads and Store Instruction visits are just for finding
GetElementPtrConstantExpr and splitting them.
- Allocas

[DirectX] Flatten arrays (#114332)

- Relevant piece is `DXILFlattenArrays.cpp`
- Loads and Store Instruction visits are just for finding
GetElementPtrConstantExpr and splitting them.
- Allocas needed to be replaced with flattened allocas.
- Global arrays were similar to allocas. Only interesting piece here is
around initializers.
- Most of the work went into building correct GEP chains. The approach
here was a recursive strategy via `recursivelyCollectGEPs`.
- All intermediary GEPs get marked for deletion and only the leaf GEPs
get updated with the new index.

fixes [89646](https://github.com/llvm/llvm-project/issues/89646)

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