The loop peeling optimization requires all loop exits to be marked
with {LoopExit,LoopExitEffect,LoopExitValue} nodes in order to peel
the first loop iteration. Previously, the graph assembler only marked
the default loop exit (taken once the loop condition evaluates to
false).

This CL adds more general support, such that all exits taken inside
the loop body passed to a ForBuilder are automatically marked. We do
this by tracking the current loop nesting level and a stack of loop
headers inside the graph assembler, and creating marker nodes as needed
inside MergeState.

Bug: v8:9972,chromium:1038297
Change-Id: I1d0196ead55d6678880f8330c7cc7b8d4f2cea06
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2000740
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65861}