Consistently assume that arithmetic operations can overflow unless one can prove the opposite.

Consistently assume that arithmetic operations can overflow unless one can prove the opposite.

Previously, HDiv never had its CanOverflow flag cleared and HMod had
inverted logic (compared to HAdd, HSub and HMul). Minor cleanups on the way.

R=jkummerow@chromium.org

Committed: http://code.google.com/p/v8/source/detail?r=14644

[Jakob Kummerow, 2013-05-13 13:29:06]

lgtm

[Sven Panne, 2013-05-13 13:48:21]

Committed patchset #2 manually as r14644 (presubmit successful).