Improve the assembly code for power function with integer exponential on x64

Improve the assembly code for power function with integer exponential on x64

The change removes one unused multiply and reschedules 
the shift, multiply and jump instructions to reduce 
stall. Experiment shows it improve about 20% performance 
on x64 for exponetials from about 100 to 2000.

Committed: https://code.google.com/p/v8/source/detail?r=12535

[xqian, 2012-09-18 06:35:07]

Hi, Yang, Kasper, Yuqiang and Vyacheslav

This is a port for patch of issue 10916311. It can gain similar performance
improvement on x64. Would you please review it?

Thanks,
-Xi

[Yang, 2012-09-18 07:59:09]

On 2012/09/18 06:35:07, xqian wrote:
> Hi, Yang, Kasper, Yuqiang and Vyacheslav
> 
> This is a port for patch of issue 10916311. It can gain similar performance
> improvement on x64. Would you please review it?
> 
> Thanks,
> -Xi

LGTM. Landing.

[xqian, 2012-09-18 10:02:30]

On 2012/09/18 07:59:09, Yang wrote:
> On 2012/09/18 06:35:07, xqian wrote:
> > Hi, Yang, Kasper, Yuqiang and Vyacheslav
> > 
> > This is a port for patch of issue 10916311. It can gain similar performance
> > improvement on x64. Would you please review it?
> > 
> > Thanks,
> > -Xi
> 
> LGTM. Landing.

Thank you!
-Xi