Make floating merges respected minimum RPO of coupled phis.

R=bmeurer@chromium.org, jarin@chromium.org
TEST=cctest/test-scheduler/LoopedFloatingDiamond

Review URL: https://codereview.chromium.org/682983003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24930 ce2b1a6d-e550-0410-aec6-3dcde31c8c00