cpu: Replace fixed sized arrays in the O3 inst with variable arrays.

cpu: Replace fixed sized arrays in the O3 inst with variable arrays.

The only way to allocate fixed sized arrays which will definitely be big
enough for all source/destination registers for a given instruction is
to track the maximum number of each at compile time, and then size the
arrays appropriately. That creates a point of centralization which
prevents breaking up decoder and instruction definitions into more
modular pieces, and if multiple ISAs are ever built at once, would
require coordination between all ISAs, and wasting memory for most of
them.

The dynamic allocation overhead is minimized by allocating the storage
for all variable arrays in one chunk, and then placing the arrays there
using placement new. There is still some overhead, although less than it
might be otherwise.

Change-Id: Id2c42869cba944deb97da01ca9e0e70186e22532
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38384
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>