Implement a cheaper and faster Debugger Running mode

[Mark Lam]

With https://bugs.webkit.org/show_bug.cgi?id=122836, when in the Debugger Running mode, the LLINT and baseline JIT generated code needs to call a slow path to check if the Debugger needs to switch to Stepping mode.  We can do better.  This is how:

1. In Running mode, only call the slow path for op_debug if the op_debug "needBreaks" flag is set i.e. we need to handle a breakpoint.
2. In Stepping mode, before taking a step, the Debugger should:
    2.1 decorate the next op_debug instruction's needBreaks with a "isStepping" bit.
          e.g bit 0 == have breakpoint set here, bit 1 == stepping to this bytecode.
    2.2. for call opcodes, fix up the CallLinkInfo to force a call to the slow path.
           2.2.1 in the call slow paths, decorate the first op_debug instruction's needBreaks flag with an "isStepping" bit.
    2.3 when handling exceptions, decorate the exception handler's first op_debug instruction's needBreaks flag with an "isStepping" bit.

With this change, the debugger should be able to achieve approximately a 3x speed up over shipping Safari on the Octane benchmark.

Note: the DFG is still not enabled for the debugger i.e. the DFG will see the op_debug opcode and fail compilation.

[Radar WebKit Bug Importer]

<rdar://problem/15232333>

[Mark Lam]

This work has been folded into <https://webkit.org/b/122836>.

*** This bug has been marked as a duplicate of bug 122836 ***