[llvm] [SimplifyCFG] Simplify switch instruction that has duplicate arms (PR #114262)

[Yingwei Zheng via llvm-commits]

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@@ -7436,6 +7437,185 @@ static bool simplifySwitchOfCmpIntrinsic(SwitchInst *SI, IRBuilderBase &Builder,
   return true;
 }
 
+/// Checking whether two CaseHandle.getCaseSuccessor() are equal depends on
+/// the incoming values of their successor PHINodes.
+/// PHINode::getIncomingValueForBlock has O(|Preds|), so we'd like to avoid
+/// calling this function on each BasicBlock every time isEqual is called,
+/// especially since the same BasicBlock may be passed as an argument multiple
+/// times. To do this, we can precompute a map of PHINode -> Pred BasicBlock ->
+/// IncomingValue and add it in the Wrapper so isEqual can do O(1) checking
+/// of the incoming values.
+struct CaseHandleWrapper {
+  const SwitchInst::CaseHandle Case;
+  DenseMap<PHINode *, DenseMap<BasicBlock *, Value *>> &PhiPredIVs;
+};
+
+namespace llvm {
+template <> struct DenseMapInfo<const CaseHandleWrapper *> {
+  static const CaseHandleWrapper *getEmptyKey() {
+    return reinterpret_cast<CaseHandleWrapper *>(
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dtcxzyw wrote:

```suggestion
    return static_cast<CaseHandleWrapper *>(
```

https://github.com/llvm/llvm-project/pull/114262