Avoid creating duplicate entries for a value when merging HSimulates

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/15660003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14766 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Omit smi-check for write-barrier unless tagged.

R=mvstanton@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15848002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14767 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Always require exact maps.

R=danno@chromium.org

Review URL: https://chromiumcodereview.appspot.com/13923003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14768 ce2b1a6d-e550-0410-aec6-3dcde31c8c00