[maglev] Remove input_count from Node constructors

Change the NodeBase bitfield to be out-of-line, and initialised by
NodeBase::Allocate. This means that we don't have to thread the
input_count through the Derived constructor just to pass it back into
the NodeBase constructor, and so we can remove those arguments (plus the
opcode ones while we're at it).

Bug: v8:7700
Change-Id: I0c96db8cdd05ef106b3cfeb31c5e0d4770d13cc9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3553103Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79627}