Remove unnecessary overflow check in BuildCreateAllocationMemento.

Remove unnecessary overflow check in HGraphBuilder::BuildCreateAllocationMemento().

With this fix codegen looks like:

mov ecx,[eax+0xf]  ;;; <@52,#38> load-named-field
add ecx,0x2  ;;; <@54,#40> add-i
mov [eax+0xf],ecx  ;;; <@56,#41> store-named-field

without it there is an overflow check and jump to deopt.

x64 code looks similar, except there is an (annoying) smi-untag then int32-to-smi around the add operation.

R=bmeurer@chromium.org, hpayer@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=18255

[mvstanton, 2013-12-04 10:24:52]

Hi guys, here is the restored overflow check. Hannes, the reason we didn't see
it was because we cleared the overflow bit on the wrong instruction (the store,
not the add).
Thanks,
--Michael

[Benedikt Meurer, 2013-12-04 10:25:40]

LGTM, go ahead! :-)

[Hannes Payer (out of office), 2013-12-04 10:32:33]

LGTM, just make the comment more precise

https://codereview.chromium.org/104313003/diff/1/src/hydrogen.cc
File src/hydrogen.cc (right):

https://codereview.chromium.org/104313003/diff/1/src/hydrogen.cc#newcode2595
src/hydrogen.cc:2595: // This smi value is reset to zero after every gc,
overflow isn't a problem.
"isn't a problem since the counter is bounded by the new space size."

[Dmitry Lomov (no reviews), 2013-12-04 10:42:36]

sheriff's notice: ok to land

[mvstanton, 2013-12-04 10:46:25]

Committed patchset #2 manually as r18255.