Reland of https://chromium-review.googlesource.com/c/543042/.

Now the OSR phase is only used when OSRing from the ast graph builder.
When OSRing from Turbofan, the implementation is now in the graph
building phase, at the beginning of the VisitBytecode function.
We are no longer generating any OSRLoopEntry or OSRNormalEntry nodes,
nor nodes for the possible code of the OSRed function which is before
the OSRed loops.

The trimming and reducing of the OSR phase is not done either. This
change in the way the way the OSR is done enabled to remove the
workaround to the bug mentioned below.

Bug: v8:6112
Bug: v8:6518
Change-Id: Ia02f2138f54fc79cab2f02fed68d9bb522d6ce14
Reviewed-on: https://chromium-review.googlesource.com/584756
Commit-Queue: Alexandre Talon <alexandret@google.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46899}