Revert "[maglev] Remove input_count from Node constructors"

This reverts commit 2ee36e4c.

Reason for revert: https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux64%20UBSan/20570/overview

Original change's description:
> [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/+/3553103
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#79627}

Bug: v8:7700
Change-Id: Ice38908e85f2980dbbe66c61fab17326b3d0be41
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3553005
Owners-Override: Tobias Tebbi <tebbi@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79631}