Various behaviour preserving changes that make it easier to switch on
callee-saved registers without having to land refactoring code at the
same time.

- Use MaybeStoreRegisters / MaybeRestoreRegisters
- Use CallRecordWriteStubSaveRegisters everywhere for now. Eventually
  this will be replaced by CallRecordWriteStub in places with fixed
  registers.
- Use WriteBarrierDescriptor::ComputeSavedRegisters, which for now
  returns the same as allocatable_registers

Full x64 implementation: https://crrev.com/c/2922604

Bug: v8:11420
Change-Id: I04e6ac2f6333edc91cb1030a0217f59ad441a1d3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2922250Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74876}