This CL merges nested loops that share the same header offset with its
parent loop, by not emitting JumpLoop bytecode for these inner loops.
Instead, we generate a Jump to its parent's JumpToHeader (which in
turn can be a JumpLoop or another Jump to its parent's JumpToHeader).

Originally, every loop had a unique first Bytecode to jump to. Since
IterationBody StackChecks are going to become implicit this will no
longer be the case.

As a note, this CL just sets the foundation that the follow-up CLs
will build on top of. Since we have explicit StackChecks, and they
are at the beginning of loops we do not have nested loops as of now.

Bug: v8:10149, v8:9960
Change-Id: I6daee4d2c6d6216f022228c87c4aa74e163997b2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2062390
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66626}