This CL introduces explicit LoopExit control nodes at loop exits.
We also attach explicit value renames (LoopExitMarker) and effect
rename (LoopExitEffect) to each loop exit. This is in preparation
to loop peeling, which will replace LoopExit, LoopExitMarker and
LoopExitEffect with Merge, Phi and EffectPhi respectively.

At the moment, we insert loop exit at every return, break,
continue and locally caught throw. We do not yet handle
uncaught throws (including error throws, such as ReferenceError).

Review-Url: https://codereview.chromium.org/2140673007
Cr-Commit-Position: refs/heads/master@{#37769}