This is to avoid calling AssembleTailCallBeforeGap and AssembleTailCallAfterGap
directly where possible (so making the tests less dependent on the code generator
interface when we're not directly testing it). It also makes sure that the
instruction we pass to AssembleTailCallBeforeGap and AssembleTailCallAfterGap is
indeed a tail call, with the immediate argument that specifies the stack delta.

This is to prepare for padding arguments for arm64 JSSP removal. We will need to
store padding in AssembleTailCallAfterGap, which will need the information from
a TailCall instruction.

Bug: v8:6644
Change-Id: Ia5485412a4244c7b2a133aa0541b9f8285680de4
Reviewed-on: https://chromium-review.googlesource.com/806117Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Cr-Commit-Position: refs/heads/master@{#49833}