Remove the pattern of calling 'Find' followed by 'Set' for IdentityMap,
with a single 'FindOrInsert' that explicitly returns whether an existing
entry was found, or the entry was inserted. This replaces 'Get', which
would return either an initialised or uninitialised entry (and callers
would rely on default initialisation to check this).

Also replace 'Set' with 'Insert', which explicitly requires that the
element didn't exist before. This matches expectations where it was
used (where those weren't replaced wholesale with 'FindOrInsert'), and
makes the naming consistent with 'FindOrInsert'.

Change-Id: I8fb76f4ac14fb92b88474965aafb1ace5fb79145
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2443135
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70300}