Replaced unary negation by multiplication with -1.

This fixes a deopt loop in the Epic Citadel demo and removes some code. Apart from that, this change is performance-neutral.

When we do something similar for BIT_NOT, the whole UnaryOp stuff can go away.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/21782002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16029 ce2b1a6d-e550-0410-aec6-3dcde31c8c00