Speed up CalledAsConstructor....

Speed up CalledAsConstructor.
Changed CalledAsConstructor to process the raw runtime stack directly,
rather than using a StackFrameIterator.  Using an interator turns out
to be quite expensive and the vast majority of the work done is not
relevant to deciding if we've been called as a constructor.

This speeds up getElementById by ~13%.


Committed: http://code.google.com/p/v8/source/detail?r=2578

[Kasper Lund, 2009-07-29 12:34:37]

STV. Another approach, which would be quite a bit harder to get right, would be
to use the custom construct code slot we have in functions, but I think we
should wait with that.