Changes

|Feature implementation notes=* Immediate worklist:# Bailouts** Prototype model backend ([https://bugzilla.mozilla.org/show_bug.cgi?id=646923 bug 646923])** Design and implement new IRs ([https://bugzilla.mozilla.org/show_bug.cgi?id=650181 bug 650181])test** Create a code generator and simple register allocator* Long-term worklist:** # Implement method inlining** # Implement advanced linear-scan register allocation** # Implement loop invariant code motion** # Implement constant sub-expression elimination** # Implement type-profiling** # Implement enough functionality to replace existing JITs** # Implement proper memory management of code** # Implement interval analysis