Reviewed by Cameron Zwarich.
        
        Store the callee CodeBlock, not the caller CodeBlock, in the call frame
        header. Nix the "codeBlock" local variable, and access the callee
        CodeBlock through the call frame header instead.
        
        Profit: call + return are simpler, because they don't have to update the
        "codeBlock" local variable.
        
        Because CTI keeps "r" in a register, reading the callee CodeBlock relative
        to "r" can be very fast, in any cases we care to optimize. Presently,
        no such cases seem important.
        
        Also, stop writing "dst" to the call frame header. CTI doesn't use it.
        
        21.6% speedup on empty function call benchmark.
        3.8% speedup on SunSpider --v8.
        2.1% speedup on v8 benchmark.
        0.7% speedup on SunSpider (6% speedup on controlflow-recursive).
        
        Small regression in bytecode, because currently every op_ret reads the
        callee CodeBlock to check needsFullScopeChain, and bytecode does not
        keep "r" in a register. On-balance, this is probably OK, since CTI is
        our high-performance execution model. Also, this should go away once
        we make needsFullScopeChain statically determinable at parse time.

        * VM/CTI.cpp:
        (JSC::CTI::compileOpCall): The speedup!
        (JSC::CTI::privateCompileSlowCases): ditto

        * VM/CTI.h:
        (JSC::): Fixed up magic trampoline constants to account for the nixed
        "codeBlock" argument.
        (JSC::CTI::execute): Changed trampoline function not to take a "codeBlock"
        argument, since codeBlock is now stored in the call frame header.
        
        * VM/Machine.cpp: Read the callee CodeBlock from the register file. Use
        a NULL CallerRegisters in the call frame header to signal a built-in
        caller, since CodeBlock is now never NULL.

        * VM/Machine.h: Made some stand-alone functions Machine member functions
        so they could call the private codeBlock() accessor in the Register
        class, of which Machine is a friend. Renamed "CallerCodeBlock" to
        "CodeBlock", since it's no longer the caller's CodeBlock.

        * VM/RegisterFile.h: Marked some methods const to accommodate a 
        const RegisterFile* being passed around in Machine.cpp.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@36997 268f45cc-cd09-0410-ab3c-d52691b4dbfc