This patch includes 3 fixes for veneers emission.

1) Block veneer pools emission in the PatchingAssembler.
2) Fix the check for veneer pool emission just before a constant pool.
3) Forbid copy of labels. The list of JumpTableEntry used to track the
   deoptimization table entries would make copies of the labels when growing.
   Doing so, it would confuse the Assembler that was tracking the labels via
   pointers.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/200133002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19941 ce2b1a6d-e550-0410-aec6-3dcde31c8c00