Reland "More aggressive reuse of spill slots in the register allocator."

This relands r23532 (https://codereview.chromium.org/310003003). Flakes seem unrelated.

TBR=titzer@chromium.org

Review URL: https://codereview.chromium.org/522173003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23536 ce2b1a6d-e550-0410-aec6-3dcde31c8c00