Remove dedicated LIR for_loop node

AGENTS.md

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+# AGENTS
+
+- Workarounds are categorically forbidden in this code base.
+- Fallbacks are categorically forbidden in every stage of compilation other than specifically parsing and error reporting.
+- Heuristics are categorically forbidden in every stage of compilation other than specifically parsing and error reporting.
+- Every compiler stage other than specifically parsing and error reporting must consume explicit facts produced by earlier stages rather than trying to recover, guess, reconstruct, approximate, or "best effort" its way to missing information.
+- Backends are categorically forbidden from thinking about reference counting in any way other than specifically dumbly following the explicit LIR `incref` and `decref` statements emitted by earlier compilation steps.