misc: Linearlize VecElem indexing.

These registers used to be accessed with a two dimensional index, with
one dimension specifying the register, and the second index specifying
the element within that register. This change linearizes that index down
to one dimension, where the elements of each register are laid out one
after the other in sequence.

Change-Id: I41110f57b505679a327108369db61c826d24922e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49148
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
26 files changed