[compiler] Template methods in SimplifiedLowering pt. 1

This is the first in a string of CLs that template SimplifiedLowering.
It is done with the purpose of decoupling the implementation in the
three phases that SimplifiedLowering has (Propagate, Retype, Lower).

Template:
 * VisitNodes
 * SetOutput
 * ProcessInput
 * ProcessRemainingInputs

Bug: v8:10424
Change-Id: I2d55148f5a4aafae5ec54d58a6690fc755806340
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2126916
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67385}