Fix worst-case behavior of MergeRemovableSimulates().

Fix worst-case behavior of MergeRemovableSimulates().

Currently, when a long series of removable simulates are merged, we do
this by merging them one by one as we find them.  As we merge the value
value lists of the simulates, those lists snowball so that we get a
quadratic complexity wrt runtime and memory consumption.

Instead, we gather simulates that need to be merged, and merge them
backwards starting from the last simulate.

R=jkummerow@chromium.org
BUG=v8:2612
Committed: https://code.google.com/p/v8/source/detail?r=14169

[Jakob Kummerow, 2013-04-04 17:12:22]

LGTM. Thanks for fixing this!

https://chromiumcodereview.appspot.com/13649003/diff/1/src/hydrogen-instructi...
File src/hydrogen-instructions.cc (right):

https://chromiumcodereview.appspot.com/13649003/diff/1/src/hydrogen-instructi...
src/hydrogen-instructions.cc:2001: list->at(j)->DeleteAndReplaceWith(NULL);
nit: s/list->at(j)/from/

[Yang, 2013-04-05 08:11:38]

On 2013/04/04 17:12:22, Jakob wrote:
> LGTM. Thanks for fixing this!
> 
>
https://chromiumcodereview.appspot.com/13649003/diff/1/src/hydrogen-instructi...
> File src/hydrogen-instructions.cc (right):
> 
>
https://chromiumcodereview.appspot.com/13649003/diff/1/src/hydrogen-instructi...
> src/hydrogen-instructions.cc:2001: list->at(j)->DeleteAndReplaceWith(NULL);
> nit: s/list->at(j)/from/

I made some small changes. Please take another look. And ignore the huge block
of new code in hydrgen.cc due to rebasing.

[Jakob Kummerow, 2013-04-08 17:09:59]

LGTM with a comment.

https://chromiumcodereview.appspot.com/13649003/diff/5001/src/hydrogen.cc
File src/hydrogen.cc (right):

https://chromiumcodereview.appspot.com/13649003/diff/5001/src/hydrogen.cc#new...
src/hydrogen.cc:3430: // Simply remove all accumulated simulates without
merging.
Please add to this comment:

This is safe because simulates after instructions with side effects are never
added to the merge list.

[Yang, 2013-04-08 17:37:32]

Committed patchset #2 manually as r14169 (presubmit successful).