cpu: Track flat register indices in the Minor CPU with a vector.

cpu: Track flat register indices in the Minor CPU with a vector.

That avoids having to know the maximum number of dest registers there
can be in any instruction, and will likely not affect the performance of
the Minor CPU overall.

Change-Id: I4e49695ba06365d52eb4ce128d5cbb30db665bd7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38387
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>