Only assign environments when they are actually needed. (x64 only)

Twin of https://code.google.com/p/v8/source/detail?r=20231.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/211153003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20271 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
ARM64: fix DoDeferredTaggedToI

BUG=

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/211063004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20272 ce2b1a6d-e550-0410-aec6-3dcde31c8c00