Tweak HConstant::EmitAtUses() to eliminate useless constant generation

Tweak HConstant::EmitAtUses() to eliminate useless constant generation

BUG=2881
R=danno@chromium.org

Committed: http://code.google.com/p/v8/source/detail?r=16731

[danno, 2013-09-13 07:37:21]

Please see my comment in the bug, I don't think the change is necessary once
titzer's fix is landed and probably already mitigated by the change in
EmitAtUses that I suggest.

[weiliang.lin2, 2013-09-13 08:42:28]

On 2013/09/13 07:37:21, danno wrote:
> Please see my comment in the bug, I don't think the change is necessary once
> titzer's fix is landed and probably already mitigated by the change in
> EmitAtUses that I suggest.

Yes, after reading your comments, I generate another much simpler patch(patch
set 2). I think it is safe to add UseCount() == 0 branch here.

[danno, 2013-09-13 09:50:32]

lgtm as long as you verify that this does not regress Octane, Kraken or
SunSpider.

[weiliang.lin2, 2013-09-13 17:13:10]

On 2013/09/13 09:50:32, danno wrote:
> lgtm as long as you verify that this does not regress Octane, Kraken or
> SunSpider.

List Octane, SunSpider and Kraken benchmarks data on my x86 PC here.  

	        ocatne	SunSpider  Kraken
ia32-new	15615	215.5ms	   1777.6ms
ia32-old	15652	217.1ms	   1778.4ms
			
x64-new	        15083	215.7ms	   1730.2ms
x64-old	        15002	215.4ms	   1729.7ms

It shows there is no any regression after this patch. Actually I could find
there is always consistent ~2 ms improvement for SunSpider on IA32 (about 1%
speed-up).

[danno, 2013-09-16 12:46:02]

Committed patchset #2 manually as r16731 (presubmit successful).