And other related cleanups (additional asserts, use named register
constants instead of the raw register name where appropriate).

Drive-by: Unset kOffHeapTrampolineRegister (it's unused on ia32).

Bug: v8:6666
Change-Id: Id2b94e0b9dbfa5d3bbacc80ec7424d38c4145658
Reviewed-on: https://chromium-review.googlesource.com/1185011
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55328}