Add a concept of "register snapshots" which snapshot the end-state
of the register allocation for a node (i.e. the state of the register
allocation when the node's code completes). These can be requested by
nodes, so that they know which registers need to be kept alive by the
node, and which of those are tagged.

Nodes can then use this information to temporarily spill registers
across a deferred call, without requiring the register allocator to
spill them unconditionally on the non-deferred path. The maglev
safepoint table has support for these additional spilled registers.

Bug: v8:7700
Change-Id: Id0052b5da86dd263f9019b1433fe5994a472a5b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3751203
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81614}