We're not allowed to modify the input register and have to
use a temporary instead, otherwise the result of expressions
containing Math.round can be wrong.

BUG=106351
TEST=test/mjsunit/compiler/regress-106351.js
Review URL: http://codereview.chromium.org/8833007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10190 ce2b1a6d-e550-0410-aec6-3dcde31c8c00