The forced allocation of fixed registers can override the arbitrary
choice of register in a previous allocation. Fix this by first
allocating fixed registers, and only afterward allocating arbitrary
registers.

Also add a DCHECK after input assignment that input locations match
their node's current location.

Bug: v8:7700
Change-Id: I262c2a1f9a3c47d5c23c84b3764569692f18f39d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3644958
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80601}