Saves and restores the dispatch pointer during calls to enable the debugger to
switch the dispatch table used by a function during it's execution.

Also moves the accumulator and context nodes to be Variables so that they will
be properly merged across branches.

BUG=v8:4280,v8:4690
LOG=N

Review URL: https://codereview.chromium.org/1684073002

Cr-Commit-Position: refs/heads/master@{#33894}