Add a genuine unary minus instruction to Crankshaft.

Add a genuine unary minus instruction to Crankshaft.

This change introduces an instruction for negation instead
of generating a multiplication with -1.

The code for x64 and ARM is not included in this change.

Review URL: http://codereview.chromium.org/6461021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6748 ce2b1a6d-e550-0410-aec6-3dcde31c8c00