[IR] Fix Module::setModuleFlag for uniqued metadata (#164580)

`Module::setModuleFlag` is supposed to change a single module. However,
when an `MDNode` has the same value in more than one module in the same
`LLVMContext`, such `MDNode` is shared (uniqued) across all of them.
Therefore `MDNode::replaceOperandWith` changes all modules that share
the same `MDNode`.

This used to cause problems for #86212, where a module is marked as
"upgraded" via a module flag. When this flag is shared across multiple
modules, all of them are marked, yet some may not have been processed at
all.

After the patch we now construct a new `MDNode` and replace the old one.

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index 30b5e48..e19336e 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -403,9 +403,14 @@ void Module::setModuleFlag(ModFlagBehavior Behavior, StringRef Key,
                            Metadata *Val) {
   NamedMDNode *ModFlags = getOrInsertModuleFlagsMetadata();
   // Replace the flag if it already exists.
-  for (MDNode *Flag : ModFlags->operands()) {
+  for (unsigned i = 0; i < ModFlags->getNumOperands(); ++i) {
+    MDNode *Flag = ModFlags->getOperand(i);
     if (cast<MDString>(Flag->getOperand(1))->getString() == Key) {
-      Flag->replaceOperandWith(2, Val);
+      Type *Int32Ty = Type::getInt32Ty(Context);
+      Metadata *Ops[3] = {
+          ConstantAsMetadata::get(ConstantInt::get(Int32Ty, Behavior)),
+          MDString::get(Context, Key), Val};
+      ModFlags->setOperand(i, MDNode::get(Context, Ops));
       return;
     }
   }