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}