Do not generate LDummyUse instruction for HCapturedObject

LDummyUse confuses the register allocator (since there is no definition
for the use).

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/222283002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20461 ce2b1a6d-e550-0410-aec6-3dcde31c8c00