More aggressive reuse of spill slots in the register allocator.

Two improvements:
- At phi nodes, we now try to merge operands into one spill slot and
  then reuse the spill slot for the phi.
- After register allocation we merge disjoint spill slots.

With this change, I see about 4% improvement in zlib.

BUG=
R=titzer@chromium.org

Review URL: https://codereview.chromium.org/310003003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23532 ce2b1a6d-e550-0410-aec6-3dcde31c8c00