[llvm] [SandboxIR] IR Tracker (PR #99238)

[Arthur Eubanks via llvm-commits]

================
@@ -51,3 +51,21 @@ For example, for `sandboxir::User::setOperand(OpIdx, sandboxir::Value *Op)`:
 - We get the corresponding LLVM User: `llvm::User *LLVMU = cast<llvm::User>(Val)`
 - Next we get the corresponding LLVM Operand: `llvm::Value *LLVMOp = Op->Val`
 - Finally we modify `LLVMU`'s operand: `LLVMU->setOperand(OpIdx, LLVMOp)
+
+## IR Change Tracking
+Sandbox IR's state can be saved and restored.
+This is done with the help of the tracker component that is tightly coupled to the public Sandbox IR API functions.
+Please note that nested saves/restores are currently not supported.
+
+To save the state and enable tracking the user needs to call `sandboxir::Context::save()`.
+From this point on any change made to the Sandbox IR state will automatically create a change object and register it with the tracker, without any intervention from the user.
+The changes are accumulated in a vector within the tracker.
+
+To rollback to the saved state the user needs to call `sandboxir::Context::revert()`.
+Reverting back to the saved state is a matter of going over all the accumulated changes in reverse and undoing each individual change.
+
+To accept the changes made to the IR the user needs to call `sandboxir::Context::accept()`.
+Internally this will go through the changes and run any finalization required.
+
+Please note that after a call to `revert()` or `accept()` tracking will stop.
+So the user would need to start it again if needed with a call to `save()`.
----------------
aeubanks wrote:

```suggestion
To start tracking again, the user needs to call `save()`.
```

https://github.com/llvm/llvm-project/pull/99238