Fix issue 979.

Fix issue 979.

The issue is caused when deoptimizing to an internal AST ID in a
postfix increment or decrement operation on variable.  This could
happen for a global variable.  In that case, the optimized code was
not properly simulating an extra stack slot in the unoptimized code to
hold the original value.

Committed: http://code.google.com/p/v8/source/detail?r=6025

[Mads Ager (chromium), 2010-12-15 08:48:50]

LGTM!