[compiler] Template methods in SimplifiedLowering pt. 2

This is a CL 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:
 * VisitInputs
 * EnqueueInputs
 * Finish ProcessRemainingInputs, which ends up templating some VistXXX
   methods

Bug: v8:10424
Change-Id: I10a22ca9a2e472d3177e910c640507cab861846d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2135636Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67390}