~~ is commonly used to truncate a value to int32 (ToInt32).

This change avoid actually emitting the bitwise operations, and
just truncates the subexpression of ~~.

BUG=v8:2037
TEST=test/mjsunit/compiler/optimize-bitnot.js
Review URL: https://chromiumcodereview.appspot.com/10123007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11390 ce2b1a6d-e550-0410-aec6-3dcde31c8c00