Make 'with' mark only variables occurring in the body as used.

Make 'with' mark only variables occurring in the body as used.

Before, we conservatively marked every variable in a scope as used if the
scope contained 'with'.  Instead, just mark the variables occurring in the
body of the with.  This avoids marking 'arguments' as used whenever 'with'
occurs, which incurs an extra performance penalty (a use of arguments is
seen as an instruction to redirect all parameter accesses to the arguments
object).

Committed: http://code.google.com/p/v8/source/detail?r=6415

[Mads Ager (chromium), 2011-01-20 09:17:36]

LGTM