[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](#89646)

Author: farzonl

llvm/lib/Target/DirectX/CMakeLists.txt

@@ -22,6 +22,7 @@ add_llvm_target(DirectXCodeGen
   DXContainerGlobals.cpp
   DXILDataScalarization.cpp
   DXILFinalizeLinkage.cpp
+  DXILFlattenArrays.cpp
   DXILIntrinsicExpansion.cpp
   DXILOpBuilder.cpp
   DXILOpLowering.cpp