[llvm] [DirectX] Do not flatten GEP chains for unsupported types (PR #150484)

[Deric C. via llvm-commits]

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@@ -263,8 +263,13 @@ bool DXILFlattenArraysVisitor::visitGetElementPtrInst(GetElementPtrInst &GEP) {
   // merge the byte offsets. Otherwise, this GEP is itself the root of a GEP
   // chain and we need to deterine the root array type
   if (auto *PtrOpGEP = dyn_cast<GEPOperator>(PtrOperand)) {
-    assert(GEPChainInfoMap.contains(PtrOpGEP) &&
-           "Expected parent GEP to be visited before this GEP");
+
+    // If the parent GEP was not processed, then we do not want to process its
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Icohedron wrote:

I could throw an error when there is an unsupported type, but would the DXILFlattenArrays pass really be the place to throw such errors?

Line 296 and below already has
```c++
    // If the root type is not an array, we don't need to do any flattening
    if (!isa<ArrayType>(RootTy))
      return false;
```
to indicate that only arrays are applicable to this transformation. 
The name of the pass "DXIL Flatten Arrays" to me also implies it only applies to arrays.

https://github.com/llvm/llvm-project/pull/150484