The mid-tier register allocator already did some consistency checks;
this CL extends them, and removes a redundant check.
The added check ensures that no two virtual registers are assigned to
the same register. A separate check for the correctness of the
{allocated_registers_bits_} bitset is folded into {CheckConsistency}.
A second check that an allocated register is contained in
{allocated_registers_bits_} is removed.

R=mslekova@chromium.org

Bug: v8:12330
Change-Id: I6420eede145f88006c49e6ab16fdbeabffb8c9c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3358291Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78473}