Scale the OSR optimization threshold by loop nesting depth.

Scale the OSR optimization threshold by loop nesting depth.

For on-stack replacement (OSR), code quality is better if we optimize at
the outermost hot loop.  Track loop nesting depth and scale the
threshold based on the loop depth to prefer outer loops over inner ones.

R=fschneider@google.com

Committed: https://code.google.com/p/dart/source/detail?r=24175

[Florian Schneider, 2013-06-19 08:55:28]

lgtm

[Kevin Millikin (Google), 2013-06-19 09:21:35]

Committed patchset #1 manually as r24175 (presubmit successful).