Handle LLVM Parallelism Instructions

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LLVM has 3 instructions made for use in a parallel environment: fence, cmpxchg, and atomicrmw. There are multiple ways to handle these:

• No support. Emit a warning when it is encountered, and treat it like a no-op. This is what we currently do.
• Support, but assume single thread. For example, cmpxchg would do the compare-and-exchange, but the exchange would always be successful in the model generated,
• Support parallel programs in general. This is very hard, and I don't even know where to begin.