[compiler] SimplifiedLowering cleanups

List:
 * Create a method so Lower is encapsulated.
 * Rename phases methods to correspond to their own Phase name.
 * Move the phases methods closer to Run() and ordered them.
 * Simplify two for loops into one.
 * Remove unused method.
 * Clean up VisitCall.

Bug: v8:10424
Change-Id: Iba41f727c79a17cb0abc165ebc3141ac736dc363
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2164786Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68289}