Optimize self-recursive tail calls
This patch implements loopification optimization. It was described in "Low-level code optimisations in the Glasgow Haskell Compiler" by Krzysztof Woś, but we use a different approach here. Krzysztof's approach was to perform optimization as a Cmm-to-Cmm pass. Our approach is to generate properly optimized tail calls in the code generator, which saves us the trouble of processing Cmm. This idea was proposed by Simon Marlow. Implementation details are explained in Note [Self-recursive tail calls]. Performance of most nofib benchmarks is not affected. There are some benchmarks that show 5-7% improvement, with an average improvement of 2.6%. It would require some further investigation to check if this is related to benchamrking noise or does this optimization really help make some class of programs faster. As a minor cleanup, this patch renames forkProc to forkLneBody. It also moves some data declarations from StgCmmMonad to StgCmmClosure, because they are needed there and it seems that StgCmmClosure is on top of the whole StgCmm* hierarchy.
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