[llvm] Add EliminateNewDuplicatePHINodes function. (PR #135179)

[Nikita Popov via llvm-commits]

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@@ -175,6 +175,22 @@ bool EliminateDuplicatePHINodes(BasicBlock *BB);
 bool EliminateDuplicatePHINodes(BasicBlock *BB,
                                 SmallPtrSetImpl<PHINode *> &ToRemove);
 
+/// Check for and eliminate duplicate PHI nodes in the block. This function is
+/// specifically designed for scenarios where new PHI nodes are inserted into
+/// the beginning of the block, such when SSAUpdaterBulk::RewriteAllUses. It
+/// compares the newly inserted PHI nodes against the existing ones and if a
+/// new PHI node is found to be a duplicate of an existing one, the new node is
+/// removed. Existing PHI nodes are left unmodified, even if they are
+/// duplicates. New nodes are also deleted if they are duplicates of each other.
+/// Similar to EliminateDuplicatePHINodes, this function assumes a consistent
+/// order for all incoming values across PHI nodes in the block. FirstExistedPN
+/// Points to the first existing PHI node in the block. Newly inserted PHI nodes
+/// should not reference one another. However, they may reference themselves or
+/// existing PHI nodes, and existing PHI nodes may reference the newly inserted
+/// PHI nodes.
+bool EliminateNewDuplicatePHINodes(BasicBlock *BB,
+                                   BasicBlock::phi_iterator FirstExistedPN);
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nikic wrote:

```suggestion
                                   BasicBlock::phi_iterator FirstExistingPN);
```

https://github.com/llvm/llvm-project/pull/135179