A collection of smallish cleanups and improvements for safepoints.

Maintainability:
- The class names were not very clear; move Safepoint inside
  SafepointTableBuilder to clarify that this wrapper class is used
  during codegen.
- Rename DefinePointerSlot/DefineRegister to
  DefineTaggedStackSlot/DefineTaggedRegister for clarity.
- Use named constants instead of -1.
- DefineTaggedRegister has no connection to kNoDeoptIndex, remove
  the DCHECK and comment.
- Remove the unused kNumSafepointRegisters constant + other dead code.
- Small clarifications in CommonFrame::IterateCompiledFrame.
- Rename has_safepoint_info to uses_safepoint_table and refactor s.t.
  `stack_slots` can be used when `uses_safepoint_table == false`. In
  this case it just returns 0.

Perf:
- During codegen, represent stack slots as a growable bit vector
  instead of a list of int indices. Extend GrowableBitVector
  functionality to support the above.
- Track the minimum index instead of iterating all stack slots in
  all safepoints before encoding.

Bug: v8:7700
Change-Id: If409bc42c825d47fc0074fce51e3b963fd080806
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3483659Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79233}