Fix some out-of-line constant pool garbage collection bugs.

This CL fixes some bugs in the out of line constant pool implementation when
constant pools are GCed.  Namely:
  - Push/Pop pp register in exit frames and VisitPointer on it to ensure it is
    updated if the ConstantPoolArray is moved by GC.
  - Mark pp as a SafePoint Register for optimized functions.
  - Ensure that StandardFrame::IterateExpressions also iterates over the
    constant pool pointer in the stackframe.
  - Fix calculation of last_ptr_offset in ConstantPoolArray body iterator.
  - Make ensure that CONSTANT_POOL_ARRAY_TYPE is a pointer object InstanceType.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/123263005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18473 ce2b1a6d-e550-0410-aec6-3dcde31c8c00