[maglev] Pass complete bitfield to Node constructors

Change the NodeBase bitfield to be initialised by NodeBase::Allocate,
and threaded through Node constructors as a complete bitfield instead of
the input_count.

This means that we can centralise changes to the bitfield, as well as
the setting of the opcode.

Bug: v8:7700
Change-Id: I848369339f3e43ba85d435c14d525d63907181e8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3555773
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79656}