Tweak HConstant::EmitAtUses() to eliminate useless constant generation

BUG=2881
R=danno@chromium.org

Review URL: https://codereview.chromium.org/23811011

Patch from Weiliang Lin <weiliang.lin2@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16731 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Make HValue::ActualValue() traverse all idefs.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23691064

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16732 ce2b1a6d-e550-0410-aec6-3dcde31c8c00