Changes

# <s>'''On-Stack Replacement'''</s>## <s>What: Jump into JIT code at loop headers, possibly by profiling off-thread.</s>## <s>Work involved:</s>### <s>Explore ways to monitor hotness of JIT code.</s>### <s>Implement the ability to jump into Ion-compiled code at loop headers, possibly by inserting special OSR nodes in the IR.</s>## <s>Priority: Medium</s>## <s>Time: 2 4 weeks</s>## <s>Owner: <b>sstangl</b></s>