[llvm] [SandboxIR] Implement PHINodes (PR #101111)

[via llvm-commits]

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@@ -1290,6 +1292,97 @@ class GetElementPtrInst final : public Instruction {
 #endif
 };
 
+class PHINode final : public Instruction {
+  /// Use Context::createPHINode(). Don't call the
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Sterling-Augustine wrote:

done

https://github.com/llvm/llvm-project/pull/101111