This reverts commit a79f9031.

Reason for revert: Need to revert:
https://chromium-review.googlesource.com/c/463287/

Original change's description:
> [Interpreter] Unify approach to building interpreter handler and Turbofan stubs.
> 
> Moves interpreter-generator.cc to a similar model of building handlers as
> Turbofan stubs elsewhere, to simplify moving code between stubs / builtins and
> bytecode handlers. This removes the "__" hack from the Interpreter generator
> code.
> 
> Also make SetBytecodeOffset private to InterpreterAssembler and make 
> LdaImmutable[Current]ContextSlot and Lda[Current]ContextSlot share
> handlers since they are identical.
> 
> Change-Id: I9e91e7d37c2ea75513e4dcc3b95b4bb6517f83da
> Reviewed-on: https://chromium-review.googlesource.com/471987
> Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#44534}

TBR=rmcilroy@chromium.org,jkummerow@chromium.org,leszeks@chromium.org,ishell@chromium.org,cbruni@chromium.org,v8-reviews@googlegroups.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I5c72a8a20ab6b8fd862427e1703c883fa484a2c1
Reviewed-on: https://chromium-review.googlesource.com/474745Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44547}