1. The placement of checks for negative zero has to be computed after
all conversion instructions have been inserted. I separated the code
into its own phase.

2. GVN need to take instruction flags into account when comparing
instructions for redundancy.

Review URL: http://codereview.chromium.org/6260035

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6534 ce2b1a6d-e550-0410-aec6-3dcde31c8c00