[turbofan] move Node to vreg mapping to InstructionSelector

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/683933004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25010 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
remove a bunch of isolate::current in api.cc

R=svenpanne@chromium.org

BUG=

Review URL: https://codereview.chromium.org/686053003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25011 ce2b1a6d-e550-0410-aec6-3dcde31c8c00