Split small chunks of assembly instructions into separate functions.
This makes the code easier to follow and to maintain, especially for
register allocation.

Drive-by: simplify stack-switching test.

R=ahaas@chromium.org

Bug: v8:12191
Change-Id: Id7544a3b2d16085540d9f1863a0eabd1f72f22bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3461929Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79105}