Port a2935d63

Original commmit message:
    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.

R=rmcilroy@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=v8:4280,v8:4690
LOG=N

Review URL: https://codereview.chromium.org/1688373002

Cr-Commit-Position: refs/heads/master@{#33962}