Add a HBreak instruction for debugging

Add a HBreak instruction for debugging

BUG=
R=jkummerow@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=14691

[Jakob Kummerow, 2013-05-15 11:23:36]

LGTM if you address my comment below.

https://codereview.chromium.org/14997008/diff/7001/src/hydrogen-instructions.h
File src/hydrogen-instructions.h (right):

https://codereview.chromium.org/14997008/diff/7001/src/hydrogen-instructions....
src/hydrogen-instructions.h:1467: class HBreak: public HTemplateInstruction<0> {
I'd prefer to call this instruction HDebugBreak (and the lithium equivalent
LDebugBreak) to avoid confusion, since it doesn't have anything to do with the
JS "break" statement, whereas we already have an OS::DebugBreak() that serves
the same purpose in C++ code.

Also, a short comment here would be nice:
// Inserts an int3/stop break instruction for debugging purposes.

[oliv, 2013-05-15 14:24:59]

Committed patchset #2 manually as r14691 (presubmit successful).