- Renames StackParameterCount to ParameterSlotCount, to avoid
  confusion between slots and parameters, which are not in general
  equivalent.
- Similarly, renames StackReturnCount to ReturnSlotCount.
- Adjusts the result of GetFirstUnusedStackSlot to return one more
  than the last slot of the argument area, not including padding.
- Renames GetFirstUnusedStackSlot to GetOffsetToFirstUnusedStackSlot
  to reflect that the result is an offset from callee to caller
  frames.
- GetReturnsOffset is a little clearer, with adjustment for the
  different semantics of GetFirstUnusedStackSlot.
- Renames 'optional_padding_slot' and 'first_unused_stack_slot'
  variables in Tailcall codegen to reflect that these are offsets.

Bug: v8:9198

Change-Id: Ib73c52710dc1f3ead640d488a6fdeb605b7b665e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2700099
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73242}