Instead of sign-extending at key use, definitions that can be used as keys are sign extended immediately after the definition.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/179773002

Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20284 ce2b1a6d-e550-0410-aec6-3dcde31c8c00