LoopExitValue nodes can be used as inputs to Phis in loop optimizations.
To do this, we need to know the machine representation that needs to be
passed to the new Phi node. This CL adds a MachineRepresentation
argument to LoopExitValue nodes, as well as a helper to extract it.
Since the MachineRepresentation is not used by JS compilation, nodes
generated during JS compilation are passed kTagged as a default value.

Change-Id: I925f382d5e6988d8fad3de7a6db231e871d6ed36
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2578983
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Georg Neis (ooo until January 5) <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71798}