[llvm] [DirectX] replace byte splitting via vector bitcast with scalar (PR #140167)

[Farzon Lotfi via llvm-commits]

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@@ -349,6 +396,7 @@ class DXILLegalizationPipeline {
     LegalizationPipeline.push_back(downcastI64toI32InsertExtractElements);
     LegalizationPipeline.push_back(legalizeFreeze);
     LegalizationPipeline.push_back(removeMemSet);
+    LegalizationPipeline.push_back(legalizeGetHighLowi64Bytes);
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farzonl wrote:

This is going to be a more involved change because we already have a pass that replaces extract elements to handle the i64 indices case. I think we should add stages

https://github.com/llvm/llvm-project/pull/140167