Remove LlvmCodeGen::CastPtrToLlvmPtr

LlvmCodeGen::CastPtrToLlvmPtr embeds a pointer that points to data in the current process's memory into codegen'd IR code. Our long term goal is to share the codegen'd IR among processes working on the same fragment, which is not possible if the IR contains pointers pointing to data of a specific process. A step in making the IR independent of the process generating it is removing LlvmCodeGen::CastPtrToLlvmPtr.